Implement Issue Config (#20956)

Closes #20955

This PR adds the possibility to disable blank Issues, when the Repo has
templates. This can be done by creating the file
`.gitea/issue_config.yaml` with the content `blank_issues_enabled` in
the Repo.

options/locale/locale_en-US.ini

@@ -1272,10 +1272,12 @@ issues.new.no_assignees = No Assignees
 issues.new.no_reviewers = No reviewers
 issues.new.add_reviewer_title = Request review
 issues.choose.get_started = Get Started
+issues.choose.open_external_link = Open
 issues.choose.blank = Default
 issues.choose.blank_about = Create an issue from default template.
 issues.choose.ignore_invalid_templates = Invalid templates have been ignored
 issues.choose.invalid_templates = %v invalid template(s) found
+issues.choose.invalid_config = The issue config contains errors:
 issues.no_ref = No Branch/Tag Specified
 issues.create = Create Issue
 issues.new_label = New Label