Add gem owner add and remove in web UI of rubygems.org

gem owner -a/r command is used to add or remove owners by an email. We will add options for adding/removing gem owners via Web UI. Additionally, we will make owner add/remove events notifiable, i.e. users can opt-in to receive email notification about ownership changes of a gem.

We will also add a new flow for ownership transfers. Any gem owner looking for maintainers will be able to create ownership request. Any user can make an ownership application for any gem with less than 100,000 downloads and older than 12 months updated at time or gems which are marked as looking for maintainers. When a gem owner approves an ownership application, the requester would be added as an owner to the gem. These ownership requests will be searchable and listed on a site wide link.