add get_media_viewer api route

[86482t]

Following #1896, with this change it is now possible to get through the api what the user has selected, either in the media viewer or in a page. While I was at it, I added more useful info that was readily available to the response.

Finally closes #1583

[hydrusnetwork]

Thank you for this work. I like the idea and this looks good! I will merge now and I'll tinker with it a bit for v655. We've been pushing on multiple media viewer support recently, so I'll see if I can return a list and have 'canvas_keys' or something to identify them. I'll bet we'll eventually want to extend this with some sort of 'tell (canvas x) to (move forward)' kind of commands.

[hydrusnetwork]

Ok just as a heads up, if you use this call already, I changed it to get_media_viewers and it returns a list of dicts like this:

{
  "canvas_type" : (some number, means like 'browser' vs 'duplicate filter' etc..),
  "canvas_key" : (bunch of hex that uniquely identifies the canvas,
  "current_media" : (either null or the same results from `file_metadata` call)
}

If we end up going this way, we could tack on stuff like 'position' and 'size' too for the actual window geometry.

I'll write out some help docs for it for 655 and figure out a unit test and all that. Thanks again!