Prior to this commit we had an inline logic check on whether or not to
render a navigation link. As we are looking to rollout the feature flag
for Listings, we needed to add another somewhat complex conditional.
This commit moves the inline conditionals to a helper function, which
makes testing the logic far easier. Especially since we need to bombard
the tests with the combination of 3 different boolean checks. (And one
of those boolean checks requires even more but could be stubbed).
There should be no UI differences, as we're presently assuming the
Listings feature is enabled.
Related to forem/rfcs#291
b4b1212a33