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/web/middleware/data.go

@@ -3,7 +3,63 @@
 
 package middleware
 
-// DataStore represents a data store
-type DataStore interface {
-	GetData() map[string]interface{}
+import (
+	"context"
+	"time"
+
+	"code.gitea.io/gitea/modules/setting"
+)
+
+// ContextDataStore represents a data store
+type ContextDataStore interface {
+	GetData() ContextData
+}
+
+type ContextData map[string]any
+
+func (ds ContextData) GetData() map[string]any {
+	return ds
+}
+
+func (ds ContextData) MergeFrom(other ContextData) ContextData {
+	for k, v := range other {
+		ds[k] = v
+	}
+	return ds
+}
+
+const ContextDataKeySignedUser = "SignedUser"
+
+type contextDataKeyType struct{}
+
+var contextDataKey contextDataKeyType
+
+func WithContextData(c context.Context) context.Context {
+	return context.WithValue(c, contextDataKey, make(ContextData, 10))
+}
+
+func GetContextData(c context.Context) ContextData {
+	if ds, ok := c.Value(contextDataKey).(ContextData); ok {
+		return ds
+	}
+	return nil
+}
+
+func CommonTemplateContextData() ContextData {
+	return ContextData{
+		"IsLandingPageHome":          setting.LandingPageURL == setting.LandingPageHome,
+		"IsLandingPageExplore":       setting.LandingPageURL == setting.LandingPageExplore,
+		"IsLandingPageOrganizations": setting.LandingPageURL == setting.LandingPageOrganizations,
+
+		"ShowRegistrationButton":        setting.Service.ShowRegistrationButton,
+		"ShowMilestonesDashboardPage":   setting.Service.ShowMilestonesDashboardPage,
+		"ShowFooterVersion":             setting.Other.ShowFooterVersion,
+		"DisableDownloadSourceArchives": setting.Repository.DisableDownloadSourceArchives,
+
+		"EnableSwagger":      setting.API.EnableSwagger,
+		"EnableOpenIDSignIn": setting.Service.EnableOpenIDSignIn,
+		"PageStartTime":      time.Now(),
+
+		"RunModeIsProd": setting.IsProd,
+	}
 }