autocomplete: rework cache policy.

The previous cache policy was that all autocomplete results were cached
for a fixed 7 days. The new policy is that if autocomplete returns more
than 10 results they're cached for 24 hours, otherwise if it returns
less than 10 results they're cached for 1 hour.

The rationale is that if autocomplete returns a lot of results, then the
top 10 results are relatively stable and unlikely to change, but if it
returns less than 10 results, then the results are unstable and can be
easily changed.

We also change it so that autocomplete calls can be cached publicly.
Public caching means that HTTP requests are cached by Cloudflare. This
will ideally reduce load on the server and reduce latency for end users.
This is only safe for calls that return the same results for all users
(i.e. the results don't depend on the current user), since the cache is
publicly shared by all users. Currently username, favgroup, and saved
search autocomplete results depend on the current user, so they can't be
publicly cached.

app/controllers/autocomplete_controller.rb

@@ -8,6 +8,10 @@ class AutocompleteController < ApplicationController
     @autocomplete = AutocompleteService.new(@query, @type, current_user: CurrentUser.user, limit: @limit)
 
     @results = @autocomplete.autocomplete_results
+    @expires_in = @autocomplete.cache_duration
+    @public = @autocomplete.cache_publicly?
+
+    expires_in @expires_in, public: @public unless response.cache_control.present?
     respond_with(@results)
   end
 end