[WIP] Sir Trevor conversion

I think if wagtail is going to migrate to Sir Trevor, it should be sooner rather than later. This is a work in progress, but as a proof of concept shows it's feasible to convert to Sir Trevor building off the modal workflow.

Off the bat there are a few styling issues (i.e., the "delete" control doesn't work so well) and UI issues (How do we indicate to the user that they've selected a particular image_format? How do we use the current "link" hallo plugins?). I'm also interested in incorporating nested blocks ([like](https://github.com/madebymany/sir-trevor-js/pull/177/files) [these](https://github.com/madebymany/sir-trevor-js/issues/68), although there doesn't seem to be a perfect solution).

As far as the "link" hallo plugins, I think we could use a [controllable mixin](https://github.com/madebymany/sir-trevor-js/blob/master/src/block_mixins/controllable.js) in a custom "text" block, where links could be inserted.

Whenever an app gets converted, it should also provide some kind of migration tool to help convert the storage. For example, the image migration tool could provide a regex to extract the image id, url, etc. and convert it to the appropriate json structure.