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/controllers/settings_controller.rb

@@ -193,7 +193,11 @@ class SettingsController < ApplicationController
 
     def store_user_avatar
       io = @user.avatar_new.tempfile
-      img_data = process_avatar(io)
+      img_data = UserManager::ProcessAvatar.call(io: io)
+      if img_data.blank?
+        @user.errors.add(:avatar, "failed to process file")
+        false
+      end
       tempfile = Tempfile.create
       tempfile.binmode
       tempfile.write(img_data)
@@ -212,18 +216,4 @@ class SettingsController < ApplicationController
         @user.save
       end
     end
-
-    def process_avatar(io)
-      processed = ImageProcessing::Vips
-        .source(io)
-        .resize_to_fill(400, 400)
-        .saver(strip: true)
-        .call
-      io.rewind
-      processed.read
-    rescue Vips::Error => e
-      Sentry.capture_exception(e) if Setting.sentry_enabled?
-      Rails.logger.error { "Image processing failed for avatar: #{e.message}" }
-      nil
-    end
 end