Fixing the builder composer and home screen

Three fixes, across the builder chat and the home screen:

• Builder composer: typing an @ followed by something that matched no file left the mention menu open with nothing in it, so pressing Enter inserted a line break instead of sending your message. Enter now sends whenever that menu has no matches to offer.
• Home screen: the Get started checklist and the home customizer panel both sit in the bottom-left corner, and the checklist was painting on top of it. Right click the home screen now and the customizer opens in front, where you can reach it.
• Builder chat: images in messages you had already sent still showed the remove button that belongs to the composer. That control now only appears while the images are still attached to a draft you are writing.