refactor gateway form handlers and subscriptions

[rafd]

Re: braidchat/braid#87 (review)

I recognize that the way these handlers / subscriptions are implemented is different than the rest of the app, where handlers and subscriptions are directly defined once. Re-frame currently makes it challenging to "modularize" or subset parts of the front-end.

Some alternatives...

• we could have the handlers be under one name, and take they key as an extra parameter, but then the views would have to know that key, which is exactly the sort of implementation detail that handlers / subscriptions are supposed to separate out.

• abandon re-frame for these modules and just make all the data local to a form component

• keep as is, and refactor when re-frame modularization hits

• ???