Various feeds changes

- Hide notes, tags and related for feed items in itembox
- Add feed support for <enclosure> elements
- Add feed syncing methods for synced settings (additional work is
  needed on the sync architecture to download synced settings from the
  server)
- Change feed item clear policy to be less aggressive
- Adjust for deasyncification
- Disable translate-on-select
- Close adomasven/zotero#7, Remove context menu items from feeds

resource/schema/userdata.sql

@@ -203,7 +203,6 @@ CREATE TABLE feeds (
     lastUpdate TIMESTAMP,
     lastCheck TIMESTAMP,
     lastCheckError TEXT,
-    lastGUID TEXT,
     cleanupAfter INT,
     refreshInterval INT,
     FOREIGN KEY (libraryID) REFERENCES libraries(libraryID) ON DELETE CASCADE