Refactor database connection (#37496)

Clean up legacy copied&pasted code, introduce the unique "database
connection" function. Move migration testing helper function
PrepareTestEnv to a separate package.

By the way, remove "shadow connection secrets" tricks: showing
connection string on UI is useless

---------

Co-authored-by: Nicolas <bircni@icloud.com>

routers/init.go

@@ -134,12 +134,6 @@ func InitWebInstalled(ctx context.Context) {
 	external.RegisterRenderers()
 	markup.Init(markup_service.FormalRenderHelperFuncs())
 
-	if setting.EnableSQLite3 {
-		log.Info("SQLite3 support is enabled")
-	} else if setting.Database.Type.IsSQLite3() {
-		log.Fatal("SQLite3 support is disabled, but it is used for database setting. Please get or build a Gitea release with SQLite3 support.")
-	}
-
 	mustInitCtx(ctx, common.InitDBEngine)
 	log.Info("ORM engine initialization successful!")
 	mustInit(system.Init)