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>

services/auth/interface.go

@@ -13,7 +13,7 @@ import (
 )
 
 // DataStore represents a data store
-type DataStore middleware.DataStore
+type DataStore middleware.ContextDataStore
 
 // SessionStore represents a session store
 type SessionStore session.Store

services/auth/middleware.go

@@ -51,13 +51,11 @@ func authShared(ctx *context.Context, authMethod Method) error {
 		ctx.IsBasicAuth = ctx.Data["AuthedMethod"].(string) == BasicMethodName
 		ctx.IsSigned = true
 		ctx.Data["IsSigned"] = ctx.IsSigned
-		ctx.Data["SignedUser"] = ctx.Doer
+		ctx.Data[middleware.ContextDataKeySignedUser] = ctx.Doer
 		ctx.Data["SignedUserID"] = ctx.Doer.ID
-		ctx.Data["SignedUserName"] = ctx.Doer.Name
 		ctx.Data["IsAdmin"] = ctx.Doer.IsAdmin
 	} else {
 		ctx.Data["SignedUserID"] = int64(0)
-		ctx.Data["SignedUserName"] = ""
 	}
 	return nil
 }