Refactoring invitations to web conferences

The form is a lot better, now with a start date, an end date, and a title.
The inputs to select users by email or name where grouped in a single input.
The form decides internally if the user should receive the message via email or private message, using the user's preferences.
An .ics file is generated for the conference and sent attached to emails.

Also refactored a bit the emails in general. Split to only mailer in more than one and reorganized the views.
Added the gem premailer-rails and a default layout that can be used for all emails.
Made it only for the email send for webconf invitations for now, but the same format can be used for the rest of the emails.