Partially fix #23050

After #22294 merged, it always has a warning log like `cannot get
context cache` when starting up. This should not affect any real life
but it's annoying. This PR will fix the problem. That means when
starting up, getting the system settings will not try from the cache but
will read from the database directly.

---------

Co-authored-by: Lauris BH <lauris@nix.lv>

To avoid duplicated load of the same data in an HTTP request, we can set
a context cache to do that. i.e. Some pages may load a user from a
database with the same id in different areas on the same page. But the
code is hidden in two different deep logic. How should we share the
user? As a result of this PR, now if both entry functions accept
`context.Context` as the first parameter and we just need to refactor
`GetUserByID` to reuse the user from the context cache. Then it will not
be loaded twice on an HTTP request.

But of course, sometimes we would like to reload an object from the
database, that's why `RemoveContextData` is also exposed.

The core context cache is here. It defines a new context
```go
type cacheContext struct {
	ctx  context.Context
	data map[any]map[any]any
        lock sync.RWMutex
}

var cacheContextKey = struct{}{}

func WithCacheContext(ctx context.Context) context.Context {
	return context.WithValue(ctx, cacheContextKey, &cacheContext{
		ctx:  ctx,
		data: make(map[any]map[any]any),
	})
}
```

Then you can use the below 4 methods to read/write/del the data within
the same context.

```go
func GetContextData(ctx context.Context, tp, key any) any
func SetContextData(ctx context.Context, tp, key, value any)
func RemoveContextData(ctx context.Context, tp, key any)
func GetWithContextCache[T any](ctx context.Context, cacheGroupKey string, cacheTargetID any, f func() (T, error)) (T, error)
```

Then let's take a look at how `system.GetString` implement it.

```go
func GetSetting(ctx context.Context, key string) (string, error) {
	return cache.GetWithContextCache(ctx, contextCacheKey, key, func() (string, error) {
		return cache.GetString(genSettingCacheKey(key), func() (string, error) {
			res, err := GetSettingNoCache(ctx, key)
			if err != nil {
				return "", err
			}
			return res.SettingValue, nil
		})
	})
}
```

First, it will check if context data include the setting object with the
key. If not, it will query from the global cache which may be memory or
a Redis cache. If not, it will get the object from the database. In the
end, if the object gets from the global cache or database, it will be
set into the context cache.

An object stored in the context cache will only be destroyed after the
context disappeared.

When offline mode is true, we should set `disable_gravatar` to `true`
and `enable_federated_avatar` to `false` in system settings.

Unfortunately, #22295 introduced a bug that when set a cached system
setting, it will not affect.
This PR make sure to remove the cache key when updating a system
setting.

Fix #22332

Fix #22281

In #21621 , `Get[V]` and `Set[V]` has been introduced, so that cache
value will be `*Setting`. For memory cache it's OK. But for redis cache,
it can only store `string` for the current implementation. This PR
revert some of changes of that and just store or return a `string` for
system setting.

#18058 made a mistake. The disableGravatar's default value depends on
`OfflineMode`. If it's `true`, then `disableGravatar` is true, otherwise
it's `false`. But not opposite.

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

Change all license headers to comply with REUSE specification.

Fix #16132

Co-authored-by: flynnnnnnnnnn <flynnnnnnnnnn@github>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

Fix #19513

This PR introduce a new db method `InTransaction(context.Context)`,
and also builtin check on `db.TxContext` and `db.WithTx`.
There is also a new method `db.AutoTx` has been introduced but could be used by other PRs.

`WithTx` will always open a new transaction, if a transaction exist in context, return an error.
`AutoTx` will try to open a new transaction if no transaction exist in context.
That means it will always enter a transaction if there is no error.

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: 6543 <6543@obermui.de>

This is a performance regression from #18058

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Andrew Thornton <art27@cantab.net>

Found using
`find . -type f -name '*.go' -print -exec vim {} -c
':%s/fmt\.Errorf(\(.*\)%v\(.*\)err/fmt.Errorf(\1%w\2err/g' -c ':wq' \;`

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>