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/user_manager/process_avatar.rb

@@ -0,0 +1,21 @@
+module UserManager
+  class ProcessAvatar < UserManagerService
+    def initialize(io:)
+      @io = io
+    end
+
+    def call
+      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.warn { "Image processing failed for avatar: #{e.message}" }
+      nil
+    end
+  end
+end