Refactor web route (#24080)

The old code is unnecessarily complex, and has many misuses.

Old code "wraps" a lot, wrap wrap wrap, it's difficult to understand
which kind of handler is used.

The new code uses a general approach, we do not need to write all kinds
of handlers into the "wrapper", do not need to wrap them again and
again.

New code, there are only 2 concepts:

1. HandlerProvider: `func (h any) (handlerProvider func (next)
http.Handler)`, it can be used as middleware
2. Use HandlerProvider to get the final HandlerFunc, and use it for
`r.Get()`


And we can decouple the route package from context package (see the
TODO).

# FAQ

## Is `reflect` safe?

Yes, all handlers are checked during startup, see the `preCheckHandler`
comment. If any handler is wrong, developers could know it in the first
time.

## Does `reflect` affect performance?

No. https://github.com/go-gitea/gitea/pull/24080#discussion_r1164825901

1. This reflect code only runs for each web handler call, handler is far
more slower: 10ms-50ms
2. The reflect is pretty fast (comparing to other code): 0.000265ms
3. XORM has more reflect operations already

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