Prepare URL Rewriter trunk for move into GitLab

The URL rewriter is going to be a standard plugin in Hippo 11.2.

The current Forge SVN trunk (2.1.0-SNAPSHOT) will be GitLab master but first there have to be some alterations because of the currently outstanding, unreleased changes:

1) Move the complete module urlrewriter-plugin-settings to the non-standard settingsmanagement plugin at https://forge.onehippo.org/svn/manage_settings/trunk/
2) Remove module urlrewriter-plugin-common since it is only one class, one constant.
3) Keep the configuration location move from "/content/urlrewriter" to "/hippo:configuration/hippo:modules/urlrewriter/hippo:moduleconfig" but make sure there is a fallback in code that still looks at "/content/urlrewriter"
4) Do not bootstrap Groovy updater script 'URLRewriter-MoveConfiguration' into the queue for automatic upgrade, but just into the registry.
5) Make a 2.0.x branch in current SVN and have the documentation generated from there (needs infra team).