Update notification bubble for net admin

[PeteMall]

See attachment:update-notification.png​

[nacin]

(In [17029]) Add theme update bubble, hide redundant bubble on network admin. props PeteMall, see #15866.

[nacin]

I was going to remark it looked odd that the update-core.php is install_plugins. But it's the same in wp-admin/menu.php too.

But then we use update_plugins in update-core.php itself. That's a better check I think. Were's the update_core check? Don't we have that cap?

[nacin]

(In [17040]) Change this string now so we can freeze earlier. see #15866.

[nacin]

Cap check fixes. Untested.

[nacin]

Patch moves to update_core for update-core.php. Also checks update_plugins for both display and the action, and update_themes for display and the action.

Plugin/theme updates can already take place on those respective pages, so I am not concerned about someone who might have update_plugins and/or update_themes but not update_core from now seeing this page -- at least for 3.1. Again, that'd have to be a plugin anyway. None of our constants split the three update_ caps from each other.

I committed a perfectly generic string that way translators can get going on it.

[ryan]

Looks good. Reviewed but haven't tested yet.

[nacin]

Tested. Denied one, two or three of the caps. Checked both the UI and the post handlers.

[nacin]

(In [17048]) Use the update_core, update_plugins and update_themes caps in update-core.php. fixes #15866.