Add project: don't show disabled organizations

[humitos]

In the first step we may want to exclude organizations that are disabled.

Same question than in #466, how do we communicate there are organization that are not listed in the first step? cc @agjohnson

[agjohnson]

It might be nice to show disabled organizations in the list with some error/disabled state, but this will take some richer listing display like what we have in the project creation repository selection view -- multiple lines for each search result, etc.

[humitos]

I went ahead and added these two checks in the validation of the form, so we don't allow the user to move forward to the next step and immediately communicate to them what's the problem: https://github.com/readthedocs/readthedocs-corporate/pull/1861

I'm leaving the richer listing display to the next iteration of this work. I opened #468 to track that work.