[web] introduce keyserver selection modal

Summary:
This diff introduces the barebones keyserver selection modal. The focus of this diff is just to implement the interface. Subsequent diffs will handle the behavior of disconnect/removing a keyserver from your keyserver selection list

Figma:

Linear task: https://linear.app/comm/issue/ENG-5497/introduce-remove-keyserver-ui

Depends on D9918

Test Plan:
Please see the demo video below

Reviewers: inka, rohan, michal

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9920