[1.x] [extensibility] feat: provide an 'actions' dropdown for extensions to add their additional buttons to the admin UserListPage

[imorland]

Changes proposed in this pull request:
Removes the Edit User button, and creates a new Actions dropdown, the old Edit User is located in here.

This allows a convenient place for 3rd party extensions to add additional actions for the user, without continuously expanding the grid. GDPR Actions for example can now be placed in this new dropdown...

Reviewers should focus on:

Screenshot

QA

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.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)