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>

routers/api/v1/misc/markup_test.go

@@ -19,6 +19,7 @@ import (
 	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/web"
+	"code.gitea.io/gitea/modules/web/middleware"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -36,7 +37,7 @@ func createContext(req *http.Request) (*context.Context, *httptest.ResponseRecor
 		Req:    req,
 		Resp:   context.NewResponse(resp),
 		Render: rnd,
-		Data:   make(map[string]interface{}),
+		Data:   make(middleware.ContextData),
 	}
 	defer c.Close()