Refactoring invitations to webconferences, first step

* Cleaned up the invitation form and added controls to set a start and end
  date for the meeting (not functional yet).
* Moved the logic from InvitesController to CustomBigbluebuttonRoomsController
  and removed invitation to events.
* Added gem 'premailer-rails', so we can format emails with inline css
* Added a default layout for emails. For now only invitations to meetings
  are using this layout.
* Added a new method in mailer/notifier.rb to create a default email. Should
  be used in the future to refactor the other methods.
* Using 'valid_email' from a fork that allows it to be used outside of modals
Add datetime pickers on invitations to webconferences

Using the new gems installed: bootstrap-datetimepicker-rails and momentjs-rails

The view and the js logic are done. Still have to capture the values on the
server and use them in the invitation messages.
Send dates when sending invitations and improve the email

Sending the start and end date in the invitation email to meetings.
Also made the email look a bit better.
Allow filtering of users by email in /users/select

Used especially when inviting people to a conference, but not only.
First version of an .ics sent attached to email invitations

Sent when inviting someone to a conference.
Improve the format and usability of dates in webconf invitations

The format now uses i18n.
The process of changing the dates is now more natural: duration is kept
in some cases, inputs are adjusted automatically, etc.
Add class and default method to show/get user timezones

Just wrapped the timezones around a class and a method to show timezones in
the views. Added the timezone to the webconf invitation form.
Fix the timezones for webconf invitations

Make it consistent. The user configures the invitation using his timezone,
and the emails are sent using the target user's timezone.
The .ics file was also fixed, now sending the dates always using UTC.
Add a title to webconf invitations

Used mostly for the .ics file. The title becomes the title of the event in
the .ics.
Refactor webconf invitations, move code to Mconf::Invitation

Cleaning up CustomBigbluebuttonRoomsController and Notifier a bit, moving
the code to Mconf::Invitation. Not 100% yet, but a lot better than before.
Now private messages for webconf invitations are being sent again.

Also added a custom mime type :pm to use when defining partials that will be
rendered into private messages.
Clean up the controller action to send webconf invitations

A lot cleaner now and doing only what it should do: deal with the parameters
submitted by the user and creating a proper response to the user.
Moved more code to Mconf::Invitation.
Better structure for mailers, for now just for webconf invitation

With this structure we can split the Notifier in more mailer classes and
nest their views in app/mailers/views. For now only the web conference
invitation mail has been taken out of Notifier and moved to a new mailer.

refs #1179
Use the sender's time zone as a fallback in webconf invitations

If the receiving user has no time zone set, uses the time zone of the sender.

refs #1179
Better duration in webconf invitations

refs #1179
Don't ever let the end date be before the start

In webconf invitations.

refs #1179
Last small adjustments for webconf invitations

refs #1179
Merge PR #17, 'branch-v2-invitation-ics' into branch-v2