Remove unused jbuilder gem

Not sure where I was going with that.
Document current Heroku setup (which will change very soon)
Switch to a version of readline that works everywhere so far

We need an older version for the Rails console.
Switch from bootstrap-wysihtml5 to wysihtml5, and use bower for client JS

Bower, Rails and Heroku docs are here: https://coderwall.com/p/6bmygq

We were barely using bootstrap-wysihtml5's added features in the first
place, the Bower-based version didn't support Bootstrap 3, and the
bundled copy of wysihtml5 was out of date.  So let's use wysihtml5
directly.

This means losing the "insert link" feature for now, but I wanted to
clean out that that toolbar anyway.

This also means we now use stacked buildpacks on Heroku.  So far, this
actually appears to work.  I'm impressed.
Flatten less important editor buttons

We need to distinguish between buttons that are critical, and ones which
aren't.