WIP feature(upgrade): Added ElggUpgrade object.

Refs #6451.

WIP. Do not merge, but please do give feedback.

- [x] Add translation for ```item:object:elgg_upgrade```
- [x] Remove icon from ```object/elgg_upgrade``` view
- [x] Update docblock in the comment script
- [ ] Add an admin notice about the pending upgrades
- [ ] Move ```#comment-upgrade-spinner``` above ```#comment-upgrade-messages``` so the messages won't push it down