Make git clone URL could use current signed-in user (#33091)

close #33086

* Add a special value for "SSH_USER" setting: `(DOER_USERNAME)`
* Improve parseRepositoryURL and add tests (now it doesn't have hard
dependency on some setting values)

Many changes are just adding "ctx" and "doer" argument to functions.

By the way, improve app.example.ini, remove all `%(key)s` syntax, it
only makes messy and no user really cares about it.

Document: https://gitea.com/gitea/docs/pulls/138

models/unittest/testdb.go

@@ -84,6 +84,7 @@ func MainTest(m *testing.M, testOptsArg ...*TestOptions) {
 
 	setting.IsInTesting = true
 	setting.AppURL = "https://try.gitea.io/"
+	setting.Domain = "try.gitea.io"
 	setting.RunUser = "runuser"
 	setting.SSH.User = "sshuser"
 	setting.SSH.BuiltinServerUser = "builtinuser"