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>

templates/base/head_script.tmpl

@@ -6,7 +6,6 @@ If you introduce mistakes in it, Gitea JavaScript code wouldn't run correctly.
 <script>
 	window.addEventListener('error', function(e) {window._globalHandlerErrors=window._globalHandlerErrors||[]; window._globalHandlerErrors.push(e);});
 	window.config = {
-		initCount: (window.config?.initCount ?? 0) + 1,
 		appUrl: '{{AppUrl}}',
 		appSubUrl: '{{AppSubUrl}}',
 		assetVersionEncoded: encodeURIComponent('{{AssetVersion}}'), // will be used in URL construction directly