Perform the "edit" action with the original devise_invitable controller, instead of our customized friend_invitations_controller.

There is no need to customize this action.
WIP: accept invitation view, with the same format as other devise views in feedbunch.
Change view header text.
The locale change dropdown now generates URLs that keep the invitation_token param, if it's present in the current params.

This means that the accept invitation view can use the dropdown to change between locales; the invitation_token param will stay in the URL, meaning that no "invalid invitation token" errors wil be raised when changing locale.