[web] introduce add keyserver modal

Summary:
This diff introduces the add keyserver modal. The focus of this diff is just the interface, and subsquent diffs will handle implementing the behavoir of adding a keyserver to the keyserver store

for context here is the figma design:

linear task: https://linear.app/comm/issue/ENG-5496/introduce-add-keyserver-input-ui

Depends on D9892

Test Plan:
Please see screenshot below

Reviewers: inka, michal, rohan

Reviewed By: rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9916