Adds authentication via digital certificate, a new option to force users to sign in to

be able to join conferences, and bug fixes.

* [#927] Add a new option to the site to force users to sign in to be able to access a

conference. When enabled, unauthenticated users cannot access conferences. Disabled by

default.

* [#936] Allow users to sign in using a digital certificate. It's an optional authentication

method model similarly to LDAP and Shibboleth.

* Added a "guest users" module, so that users can sign in using an external authentication

method just to join a conference, without receiving a new account. Works only for the

digital certificate authentication for now.

* [#940] Fix web conference invitations when using Russian or Bulgarian.

* [#814] Fix small font sizes in some email readers. Having a `font-size` of "0.8125rem"

would cause some readers to show very small text. Removing the `font-size` lets the email

client decide the appropriate size.

* [#932] Fix the redirect when an admin of a space changes his role from admin to normal user.

* Add missing migration from BigbluebuttonRails 2.0.0 to fix the type of the column

`size` in the table `bigbluebutton_recordings`. Will only affect servers that updated

Mconf-Web very frequently and used non stable versions (very few servers).

* Update BigbluebuttonRails and adjust to its changes. Changes the way the gem is customized

and it now doesn't have direct database associations between rooms and servers, the

association is done dynamically.

* A few gem updates for compatibility with Ubuntu 16.04.

* Send web conference invitations also to the organizer. The user that is sending the

invitations will now also be in the list of attendees and receive an email with the invitation.

* Improve the design of the list of recordings (in `manage/recordings`), now it also shows the

size of the recordings.

[2.5.0]: https://github.com/mconf/mconf-web/issues?q=milestone%3Av2.5.0