Submit the forms when the enter key is pressed

@studiogestalt [mentioned on Twitter](https://twitter.com/studiogestalt/status/432697982618566656) that GuideGuide 3 no longer has the "submit on enter" feature that was present in GuideGuide 2. This one fell through the cracks during GuideGuide 3 development. It's a good feature to have, so it's back in.