Don't default user timezone to UTC.

The Mconf::Timezone class will handle the nil case by setting the site timezone
Now it's possible to set a welcome message and layout in both space and users webconferences
Correct english grammar on conference exit message
Merge remote-tracking branch 'origin/branch-v2-small-corrections' into branch-v2

Conflicts:
	config/locales/pt-br/_simple_form.yml
Improvements in setting a room's welcome message

* Shorter textarea in my/edit_room
* Better placeholders that show the default welcome message
* Reviewed the strings in the locales

After 1200a58 and f54bb90f