Unsubscription & choose to receive a mail onStart / onEnd

- The subscription panel is placed in the "mysettings" menu (not anymmore a popup)
- A Unsubscribe button were added. It removes the email if found
- The 2 options to receive a mail when someone is starting to edit and finished to edit can be chosen for each user (at least one of the two is compulsory)
- First time you go on the email notification panel in the mysettings menu, a call is made to get the email and options already subscribed for this userId
  * If found, the form is pre-filled with the datas
  * If not, the form is set with defaults values
It's just informative. Users can still subscribe another mail for this userId.
- Removed the integration of this plugin in the "Share this pad" menu. First becaue, imo, it's not the place, and second, because it meant 2 forms with the same id.
Small corrections

- put back msgs to the debug level instead of info
- remove a call to ep_etherpad-lite/node/utils/Settings as it was for a function that doesn't exist anymore
- remove checked_state var for the form as I manage myself the default check for the options.
Merge pull request #1 from quenenni/optionsAdded

Options added