[keyserver, web] Remove baseHref field

Summary:
issue: https://linear.app/comm/issue/ENG-4214/review-basehref-filed-in-web-redux
baseHref field can be merged with urlPrefix. To see the full explanation see the discussion in the linked issue.

Test Plan: Checked that web still works - connects to the keyserver. Ran yarn flow-all

Reviewers: michal, kamil

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8704