Sync Mastodon IDs/profiles to local accounts

Add a new service to import some data from Mastodon accounts:

* Find users by username, store Mastodon account ID in local db when
  found
* Import display name (don't overwrite existing)
* Import avatar (don't overwrite existing)

app/services/mastodon_manager/fetch_user.rb

@@ -0,0 +1,12 @@
+module MastodonManager
+  class FetchUser < MastodonManagerService
+    def initialize(mastodon_id:)
+      @mastodon_id = mastodon_id
+    end
+
+    def call
+      user = get "v1/admin/accounts/#{@mastodon_id}"
+      user.with_indifferent_access
+    end
+  end
+end