Allow specifying a replacement theme when deleting a custom theme

From what I have observed, when a custom theme is deleted, any exams using that theme automatically revert to the default "standard" theme.

A nice feature would be allowing theme authors to select an alternative theme to use as a replacement during the deletion process. This would allow exams using older themes, for example, to move to newer themes easily.

This would be very helpful as I am currently trying to find copies of our institution's theme and replace them with theme we consider the canonical one.

### Suggested behaviour:

My idea would be that, when a user attempts to delete a theme Numbas should:
- list the affected exams (if feasible),
- let the user select a replacement theme from the list of available themes, and
- apply the replacement theme to all affected exams.

If no replacement theme is selected, fall back to the current behaviour (revert to standard).

No files, tests, or entry points are named. Start by tracing the custom-theme deletion flow and how exams reference themes. Done means affected exams can be listed when feasible, a replacement theme can be selected and applied to all of them, and deletion still falls back to the standard theme when none is selected.

Written by the indexing model from the issue text.