Removed a couple unneeded calls to can_user_see()

I'm going to refactor this code, so I figure we should delete the
cruft first.

We don't need it inside the video or language page, since the user
shouldn't be able te view the page at all if that's the case.

Deleted can_view_version() which is not used anymore.

Removed can_see_video() check from can_view_private_version(), since
the check for membership is a stronger assertion anyways.
Replaced Video.can_user_see() with subtitle workflow method

This gives us more flexibility, since we can define different checks
for each workflow.

Added a Video.get_workflow() method that gets a workflow and caches
the result.  Refactored code in the video view decorators to use the
new system.  Removed some old/dead code while I was at it.

There was one reference to can_user_see() in the teams permissions
code.  I'd rather not touch that code too much, instead we should just
delete most of it when we get rid of tasks.  Replaced the method call
with code that does the same thing.
Merge pull request #1826 from pculture/video-view-permissions

Video view permissions