Filter out "already active" versions when activating a new version

[humitos]

If you go to https://beta.readthedocs.org/dashboard/test-builds/version/create/ to activate a version and start typing conda- it will show versions that are already active, which only introduce confusions.

I'd suggest to filter out from the list those versions that are already active.

[agjohnson]

In #287 I noted how I might address this, and did do the quick thing to just filter out active versions, but I felt the UX was equally confusing with these versions missing suddenly. I got a chance today to use custom results in the search instead and started to make the results a bit richer:

We have a bit more room to add metadata now, instead of just title/description. Active versions can show in the list still, but stand out as already activated. This makes it less confusing than outright discarding these versions from the results.