WIP: board config

What does this MR do?

Replace board switcher second page with modal

TODO

• use popup_dialog.vue
• style modal (header/footer grey)
• Close / Cancel / Click outside should all do same thing
• Milestone dropdown same style as sidebar (including Edit button)

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

#2518 (closed)