Improve template system and panic recovery (#24461)

Partially for #24457

Major changes:

1. The old `signedUserNameStringPointerKey` is quite hacky, use
`ctx.Data[SignedUser]` instead
2. Move duplicate code from `Contexter` to `CommonTemplateContextData`
3. Remove incorrect copying&pasting code `ctx.Data["Err_Password"] =
true` in API handlers
4. Use one unique `RenderPanicErrorPage` for panic error page rendering
5. Move `stripSlashesMiddleware` to be the first middleware
6. Install global panic recovery handler, it works for both `install`
and `web`
7. Make `500.tmpl` only depend minimal template functions/variables,
avoid triggering new panics

Screenshot:

<details>

![image](https://user-images.githubusercontent.com/2114189/235444895-cecbabb8-e7dc-4360-a31c-b982d11946a7.png)

</details>

1 files changed, 0 insertions, 31 deletions

diff --git a/modules/templates/base.go b/modules/templates/base.go
index 4254a56976..ef28cc03f4 100644
--- a/modules/templates/base.go
+++ b/modules/templates/base.go
@@ -5,43 +5,12 @@ package templates
 
 import (
 	"strings"
-	"time"
 
 	"code.gitea.io/gitea/modules/assetfs"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
 )
 
-// Vars represents variables to be render in golang templates
-type Vars map[string]interface{}
-
-// Merge merges another vars to the current, another Vars will override the current
-func (vars Vars) Merge(another map[string]interface{}) Vars {
-	for k, v := range another {
-		vars[k] = v
-	}
-	return vars
-}
-
-// BaseVars returns all basic vars
-func BaseVars() Vars {
-	startTime := time.Now()
-	return map[string]interface{}{
-		"IsLandingPageHome":          setting.LandingPageURL == setting.LandingPageHome,
-		"IsLandingPageExplore":       setting.LandingPageURL == setting.LandingPageExplore,
-		"IsLandingPageOrganizations": setting.LandingPageURL == setting.LandingPageOrganizations,
-
-		"ShowRegistrationButton":        setting.Service.ShowRegistrationButton,
-		"ShowMilestonesDashboardPage":   setting.Service.ShowMilestonesDashboardPage,
-		"ShowFooterVersion":             setting.Other.ShowFooterVersion,
-		"DisableDownloadSourceArchives": setting.Repository.DisableDownloadSourceArchives,
-
-		"EnableSwagger":      setting.API.EnableSwagger,
-		"EnableOpenIDSignIn": setting.Service.EnableOpenIDSignIn,
-		"PageStartTime":      startTime,
-	}
-}
-
 func AssetFS() *assetfs.LayeredFS {
 	return assetfs.Layered(CustomAssets(), BuiltinAssets())
 }