Add a new confval; `linkcheck_warn_redirects` to emit a warning when
the hyperlink is redirected.  It's useful to detect unexpected redirects
under the warn-is-error mode.

Add a new confval; linkcheck_ignore_redirects to ignore hyperlinks
that are redirected as expected.

Co-authored-by: François Freitag <mail@franek.fr>

Now linkcheck builder integrates `linkcheck_warn_redirects` into
`linkcheck_allowed_redirects`.  As a result, linkcheck builder will
emit a warning when "disallowed" redirection detected via
`linkcheck_allowed_redirects`.