[web] add keyserver url validation logic and error handling to the add keyserver modal

Summary:
This diff adds keyserver url validation logic to the onClickAddKeyserver callback in the add keyserver modal. We also now display an error message if for any reason the keyserver url inputed by the user is not valid

Linear task: https://linear.app/comm/issue/ENG-4726/dispatch-add-keyserver-redux-action-when-onpress-save-button-is

Depends on D10094

Test Plan:
Please see the demo video below

Reviewers: inka, michal, rohan

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10154