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/import_remote_avatar.rb

@@ -0,0 +1,43 @@
+module UserManager
+  class ImportRemoteAvatar < UserManagerService
+    def initialize(user:, avatar_url:)
+      @user = user
+      @avatar_url = avatar_url
+    end
+
+    def call
+      if import_remote_avatar
+        LdapManager::UpdateAvatar.call(user: @user)
+        XmppSetAvatarJob.perform_later(user: @user) if Setting.ejabberd_enabled?
+      end
+    end
+
+    private
+
+    def import_remote_avatar
+      tempfile = Down.download(@avatar_url)
+      content_type = tempfile.content_type
+      unless %w[image/jpeg image/png].include?(content_type)
+        Rails.logger.warn { "Wrong content type of remote avatar for user #{user.cn}: '#{content_type}'" }
+        return false
+      end
+
+      img_data = UserManager::ProcessAvatar.call(io: tempfile)
+      tempfile = Tempfile.create
+      tempfile.binmode
+      tempfile.write(img_data)
+      tempfile.rewind
+
+      hash = Digest::SHA256.hexdigest(img_data)
+      ext = content_type == "image/png" ? "png" : "jpg"
+      filename = "#{hash}.#{ext}"
+      key = "users/#{@user.cn}/avatars/#{filename}"
+
+      @user.avatar.attach io: tempfile, key: key, filename: filename
+    rescue => e
+      Sentry.capture_exception(e) if Setting.sentry_enabled?
+      Rails.logger.warn "Importing remote avatar failed: \"#{e.message}\""
+      false
+    end
+  end
+end

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