Also fix some tests that were broken by bumping golang and dependencies
in the previous commits.
Note that in addition to changes made to satisfy the linter which do not
impact the behavior of the code, this commit also adds ReadHeaderTimeout
to all usages of http.Server to satisfy the linter (and because it
seemed like a good suggestion).
Also:
- Add CSS to login page
- Refactor login page HTML and CSS into a new package
- New custom CSP headers for the login page, because the requirements
are different from the form_post page
