refactor(Algolia): tweak application config

[thecristen]

Summary of changes

Asana Ticket: No ticket

These are a few small changes in the Algolia backend configuration that shouldn't result in any noticeable changes.

1. use Mix.Config is deprecated, so firstly this PR migrates the configuration to use import Config
2. Nothing was using the configuration described in config :algolia, :keys, so that's deleted here 💀
3. Nothing's using the config :algolia, :index_suffix bit either, but I am planning on using it in my next PR, so I haven't deleted it.
4. I changed the index_suffix from default of "" and test value of "_test" to default of "_test" and prod value of "". This was me thinking it'd be better for us to query the routes_test, stops_test, drupal_test indexes when we're working on dev, whereas we can leave working with the routes/stops/drupal indexes to the MIX_ENV=prod environment. (Note this isn't implemented yet though, everything's still working on the non-test indexes for now)

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