Allow admins to remove restrictions

As for what's explained here we should try to make the app work even if there's no authenticated user.

It can easily be done using the user.is_authenticated method and Jinja2 if/else statements.

(See the ones that are resolved for reference) When the config option restrictPublicUsage is False and the user is NOT logged in, this pages need:

• ytsearch.html ⇾ Don't show follow/unfollow when not logged in.
• channel.html ⇾ Don't show follow/unfollow when not logged in.
• search.html ⇾ I think nothing is required. Test.
• user.html ⇾ Don't show follow/unfollow when not logged in.
• base.html ⇾ Show ONLY AVAILABLE routes on menu when restrictions are disabled.