This commit builds on the conditional rendering of navigation links by
adding a routing constraint to all :listing routes.
The impact is, if we were to disable the listing feature (e.g.,
`FeatureFlag.disable(:listing_feature_enabled)`) all requests to `GET
/listings` (and those drawn in the [config/routes/listings.rb][1] file)
would return a 404 response.
Related to forem/rfcs#291, #16335, #16338, #16362.
Testing this change:
1. Start your local application (e.g. `$ bin/startup`)
2. Go to http://localhost:3000/listings
3. You should get a Successful response.
4. Now disable the feature (e.g. `$ bin/rails r \
"FeatureFlag.disable(:listing_feature_enabled)"`)
5. Refresh http://localhost:3000/listings
6. You should get an `ActiveRecord::RecordNotFound` error; because the
application is now looking for a user or org named "listings"; in other
words we're not routing `GET "/listings"` to `listings#index` controller
action.
7. Now enable the feature (e.g. `$ bin/rails r \
"FeatureFlag.enable(:listing_feature_enabled)"`)
8. Refresh http://localhost:3000/listings
9. You should get a Successful response.
10. Goto 4
This commit is intended to be the penultimate change before we toggle
the listings section off by default.
[1]:c2ce2c32d5/config/routes/listing.rb (L1)
765df66084