Set Err_Interval in the context data so that the mirror interval box is
highlighted red as expected.
Clear Err_RepoName for the mirror and advanced actions. repo_name is not
set by these forms, causing auth.validate() to set the Err_RepoName
before SettingsPost is called, which would lead to the repository name
box getting erroneously highlighted red.
Fixes: https://github.com/go-gitea/gitea/issues/6396
909feaafa7