Add an ENV var setting for typeahead search

[kdoh]

While we've done a decent amount of performance testing for our new type ahead search for 0.39, with something like this it's always possible that someone out there could run into issues with their database given the increased potential for requests. To make sure that folks wouldn't need to downgrade in the event of this possibility, we should provide an escape hatch to turn off type ahead if their database or hosting environment doesn't support the feature well.

• This should be an environment setting with no UI.
• Let's call this setting search-type-ahead-enabled
• It should default to true.

This is my rough idea of what the defsetting method would look like

(defsetting search-type-ahead-enabled?
  (deferred-tru "Enable type ahead search in the Metabase navbar?")
  :type :boolean
  :default true)

The frontend should check if this setting is enabled and only make requests via the search result loader if it is. When this setting is false, search should work as it does in 0.38 where you type your term and get taken to the results page.