Cleanup whole handing of absolute and relative urls + remove support for proxy admin setup

This is quite a big pull request that addresses mysociety#692 and mysociety#804 and also does various code cleanups.

It
* removes support for the proxy admin setup (and removes the associated configuration in `config/general.yml`)
* Makes internal links in the app use relative links.
* Make helper method names more consistent in their use of `_path` and `_url` names
* Makes redirects use absolute urls.
* Replaces hardcoded urls with named routes

There is more code cleanup that can be done in this area, especially with the code in https://github.com/mysociety/alaveteli/blob/develop/app/helpers/link_to_helper.rb but I think this pull request is big enough as it is without further complicating it!