add pref setting to "always allow" full screen view

[bradleyrichter]

Test Plan:

1. go to about:preferences#security
2. find new fullscreen select setting, hit "always allow"
3. go to https://vimeo.com, open fullscreen
4. fullscreen should be open without notification prompt
5. Go back to fullscreen setting
6. Disable with "always ask"
7. Go to https://vimeo.com or any other video website
8. Make video fullscreen, notification should open

QA note:

1. When notification for fullscreen is set to "deny", even without "remember this decision" checkbox marked, option is stored and you can't undo. Bug wasn't introduced by this PR and was filled under Denying fullscreen mode always remember decision until browser restart #7010 with proper STR.

original issue text

Adding this option will allow a user to override the default for full-screen requests.

The default will still remain as "Always ask".

Menu choices:

"Always ask"
"Always allow"

[bradleyrichter]

I'll make a separate issue for changes to the alert bar which are waiting on this new setting.

[bsclifton]

@cezaraugusto would you be up for grabbing this one?

[abhibeckert]

Related, the "Example.com entered full screen mode. Press ESC to exit" message is annoying:

• I know I'm in full screen mode – I just clicked an "enter full screen" button.
• I know how to exit full screen – I've been doing it for years.
• The message is large and covers part of the page content and cannot be hidden, all I can do is wait for it to go away, which takes several seconds.

I think the message should be removed completely. What reason is there to keep it?