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/find_user.rb

@@ -0,0 +1,14 @@
+module MastodonManager
+  class FindUser < MastodonManagerService
+    def initialize(username:)
+      @username = username
+    end
+
+    def call
+      users = get "v2/admin/accounts?username=#{@username}&origin=local"
+      users = users.map { |u| u.with_indifferent_access }
+      # Results may contain partial matches
+      users.find { |u| u.dig(:username).downcase == @username.downcase }
+    end
+  end
+end