Review conditions for warning for missing label for a repeat

[lindsay-stevens]

• Via Remove warning for group without label #785 the warning for a group missing a label was removed.
• This issue comment stated the repeat label is "almost always useful in the UI".
• This PR comment stated "Repeats can't have media associated with them".
  • This does not seem to be explicitly mentioned or forbidden in the specs or docs
  • Currently (v4.2.0 + 122548c), a form with a repeat with a missing label but with media compiles without warnings
  • A repeat is wrapped in a group which has the label, and that group supports localisation and media
• The current warning logic is such that a missing label warning would only fire if there is no media (audio, image, video) as well:
  • So if a user puts something in audio but no label then there is no warning

if (
    control_type == constants.REPEAT
    and constants.LABEL not in row
    and row.get(constants.MEDIA) is None
):

Some options:

1. delete the third condition so that media columns are not part of the condition for the missing repeat label warning
2. do the above + add a warning if a repeat defines any media (since it will be ignored and no ODK clients support it)
3. consider whether any media types may be worth supporting for repeats in collect or webforms and if so leave as-is
4. leave it as-is and do nothing else

If any code changes are made it would be an opportunity to improve the message(s) e.g. say why it's a problem, how to fix it, link to docs, etc.