Previously the data catalog collection would be initialized
in the data catalog controller, which was appropriate since
that was the only place it would be used.

However, since now we'll need to use it in at least one more
place, namely the upcoming Monitor tab, we move it to App,
using the same pattern as for Analyze Collection.

This is a pure refactor, and does not affect BiG-CZ behavior
at all.

Adds a tab selection header to the top of the analyze sidebar,
and a placeholder for where the monitor contents will go.

Adjusts styling to better match the mockup by making the top
tab bar grey and everything else white, and accounts for it
when calculating the tab content height so that scrolling
works correctly.

This matches the mockup and demos the tab switching correctly.
It currently has no behavior.

In order to not affect BiG-CZ, it is better to create a new
set of views under /monitor rather than reuse and change the
ones under /data_catalog. We will use the same models though,
and much of the functionality will be copied over, so it is
not going to be a completely new effort.

All the styling uses flexbox.