search ui: remove cta from search contexts dropdown
Created by: limitedmage
This removes the CTA in the search context dropdown now that we don't allow users to add their own repos on dotcom. (More context in Slack.)
This also removes a ton of code associated with simply populating the data needed to render the dropdown, which is why this PR is so big.
Test plan
Verify the dropdown no longer shows CTA by running sg start dotcom and using the webapp with a logged-out user.
App preview:
Check out the client app preview documentation to learn more.