Catch duplicate labels in general configuration

[jefft]

Fixes #1305.

This PR builds upon / incorporates #1312. I don't know how to make Github show only f24004d, which is the only distinct commit not from #1312.

Jethro needs to detect duplicates in some fields on the general configuration page, namely:

• Person Status Options
• Age Bracket
• Group Membership Status Options

This PR prevents submission of duplicates:

Screencast_20250917_220124.webm

In terms of code, I've introduced a required-noduplicates class, which may be applied to the tbody of a set of labels:

<tbody class="required-noduplicates" data-noduplicates-fieldkey="age_bracket">

This follows the pattern of the existing required-checkbox-col class.

Note: duplicated blanks are caught like any other:

Users might do this under the mistaken impression that deleting a label will delete the option (it won't - there's a checkbox for that). Perhaps, as a further PR, we could have a required-nonblank class which prints a friendly "Are you trying to delete this field? Try the 'Delete' checkbox!".