[imorland]

Fixes #5263

Changes proposed in this pull request:
GroupResource sets each group name with its own Group::rename() call, and rename() raised Renamed whenever the names were dirty. So creating a group dispatched Renamed twice, and a rename from the admin's edit group modal, which sends both names, dispatched it twice too; Audit logged each one as group.renamed. rename() now raises nothing for a group that doesn't exist yet, and replaces any Renamed already pending, so a save dispatches at most one event with the original names.

Reviewers should focus on:

• New tests in core (groups\CreateTest, new groups\UpdateTest) and stricter Audit CoreGroupTest assertions; all failed before the fix.

Screenshot
N/A

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation. (No frontend changes.)
• Frontend changes: tests are green (run yarn test in js/). (No frontend changes.)
• Frontend changes: tests have been added, or are not appropriate here. (No frontend changes.)
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite).
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.