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>

modules/context/api.go

@@ -222,7 +222,7 @@ func APIContexter() func(http.Handler) http.Handler {
 			ctx := APIContext{
 				Context: &Context{
 					Resp:   NewResponse(w),
-					Data:   map[string]interface{}{},
+					Data:   middleware.GetContextData(req.Context()),
 					Locale: locale,
 					Cache:  cache.GetCache(),
 					Repo: &Repository{
@@ -250,17 +250,6 @@ func APIContexter() func(http.Handler) http.Handler {
 			ctx.Data["Context"] = &ctx
 
 			next.ServeHTTP(ctx.Resp, ctx.Req)
-
-			// Handle adding signedUserName to the context for the AccessLogger
-			usernameInterface := ctx.Data["SignedUserName"]
-			identityPtrInterface := ctx.Req.Context().Value(signedUserNameStringPointerKey)
-			if usernameInterface != nil && identityPtrInterface != nil {
-				username := usernameInterface.(string)
-				identityPtr := identityPtrInterface.(*string)
-				if identityPtr != nil && username != "" {
-					*identityPtr = username
-				}
-			}
 		})
 	}
 }