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/user/profile.tmpl

@@ -5,7 +5,7 @@
 			<div class="ui five wide column">
 				<div class="ui card">
 					<div id="profile-avatar" class="content gt-df">
-					{{if eq .SignedUserName .ContextUser.Name}}
+					{{if eq .SignedUserID .ContextUser.ID}}
 						<a class="image" href="{{AppSubUrl}}/user/settings" data-tooltip-content="{{.locale.Tr "user.change_avatar"}}">
 							{{avatar $.Context .ContextUser 290}}
 						</a>
@@ -30,7 +30,7 @@
 							{{if .ContextUser.Location}}
 								<li>{{svg "octicon-location"}} {{.ContextUser.Location}}</li>
 							{{end}}
-							{{if (eq .SignedUserName .ContextUser.Name)}}
+							{{if (eq .SignedUserID .ContextUser.ID)}}
 								<li>
 									{{svg "octicon-mail"}}
 									<a href="mailto:{{.ContextUser.Email}}" rel="nofollow">{{.ContextUser.Email}}</a>
@@ -100,7 +100,7 @@
 								</ul>
 							</li>
 							{{end}}
-							{{if and .IsSigned (ne .SignedUserName .ContextUser.Name)}}
+							{{if and .IsSigned (ne .SignedUserID .ContextUser.ID)}}
 							<li class="follow">
 								{{if $.IsFollowing}}
 									<form method="post" action="{{.Link}}?action=unfollow&redirect_to={{$.Link}}">