Admin action: Add the ability to promote users to admin

[MV-GH]

Pre-Flight checklist

• [X] Did you check to see if this issue already exists?
• [X] This is a single feature request. (Do not put multiple feature requests in one issue)
• [X] This is not a question or discussion. (Use https://lemmy.ml/c/jerboa for that)

Describe The Feature Request Below

Admin action: Add the ability to promote users to admin