COMPASS-298: Make all collection headers fixed position

[Sean-Oh]

cc: @fredtruman

[fredtruman]

👍 Overall LGTM for the moment. A couple notes though:

• I think ideally the "rule builder" and "json" view switchers on the document validation tab would also stick, i.e. not scroll with the content below, but understand that's probably not possible at the moment with the way the page is constructed.
• I imagine this issue will get slightly more complicated once we remove the query bar from the indexes and doc validation tabs, move the [insert] button to its own .action-bar row, etc. which cause a variable header height across collection views.

[Sean-Oh]

• I think we could make the rule builder / json view switchers fixed as well. I can look take a look at this in another PR.
• Once we remove the query bar, we can have two margin-top values dependent on the varying height of the header. We would just append them accordingly.

[imlucas]

Backported to 1.5-releases