Disable the creation of new tags through the UI

[akatsoulas]

New tags should be added only through the admin interface for now. In the future, a form will be created for users to suggest new tags.

[akatsoulas]

PR https://github.com/mozilla/kitsune/pull/6326

[emilghittasv]

I can confirm that:

• New question tags cannot be created for posted AAQ questions via the "Apply a tag" section.
• Existing question tags can be successfully applied to questions.
• Unable to add an empty tag via the "Apply a tag" section.
• The "Add" button has changed to "Apply" for the "Apply a tag" section.
• New tags can be successfully created via Admin and then added inside the forum questions.
• Tags can be successfully deleted via Admin and the deleted tag cannot be added inside the forum questions anymore.

I'm moving this ticket inside the release column.