use Uri.js in adhocracy.overlay.rebindCameFrom

This
* simplifies code and
* allows to replace the came_from parameter if it already exists instead
of stupidly appending.
Do dynamic patch_camefrom only if this doesn't lead to 403 (fixes #631)
Refactor login_redirect_url and friends

Simplify code and make sure URLs which take instance into account are
created. See discussion in #631.
Allow request.params in MockRequest

This fixes test_url after the parent commit.
Fixup: Return string instead of Uri object

Fixup d6728676e39a82f84bfd4fa6cf91586d16386e4f. This also worked, as
this seems to be automatically casted to a string, but better make it
explicit.
Clarify components.add_patch_camefrom_class

Fixup 19d1e181d622eff993b02cfc2624f82edd04a754
Merge pull request #637 from nidico/feature-631-fix-403-after-camefrom

Prevent came_from to redirect to 403