Allow disabling of all block_formatters on rich text fields

[timriley]

Right now passing a block_formatters: [] option to a rich text area still shows the standard block formatters:

For rich text areas where we only want to allow inline formatting, it would be helpful to be able to disable the block formatting options altogether.

[makenosound]

@timriley Ah, right, that is a bug that I imagine is pretty easy to adjust. Seems fair to use that configuration, yeah?

[timriley]

@makenosound I reckon it'd be fair to support no block formatting whatsoever, yeah. It'd be a good choice for us to use whenever we've previously opted for our "single-line" custom markdown rendering.

I guess there could be some UI implications of completely removing all the block formatters? You might want to remove the menu indicator (or maybe consider removing the left gutter altogether?)