Download Mosaico JS files via composer-downloads-plugin

[totten]

This should lighten the load on downloading from git (and also resolve #479).

• Before: Clone the main Mosaico repo. Run a full JS toolchain (npm, node, bower, grunt, phantom) to create the compiled Mosaico JS.
• After: Download pre-compiled Mosaico JS.

[totten]

This pulls from a tag on totten/mosaico. However, if we merge civicrm/mosaico#13 first, then we can download from civicrm/mosaico instead.

[mattwire]

@totten I merged civicrm/mosaico#13 can you update this PR now?

[mattwire]

Great thanks @totten I've merged. Let me know if there is anything else we need to do to make this work? Will need to update the docs for publishing each release

[totten]

FWIW, I had to re-tag civicrm/mosaico with new merge.

In terms of docs and publishing, the main thing is that we now have to treat the two repos as separate deliverables. So:

• The common case of developing+releasing changes for the integration (uk.co.vedaconsulting.mosaico) should be faster/easier now.
• The less common case of developing+releasing changes for the JS (mosaico) should be harder/slower now.
  • Development: Need to separately clone the additional git repo. Need to setup JS toolchain. etc
  • Release:
    • First, tag a new release on mosaico.git.
    • Wait 15 minutes. (It takes 14 minutes to execute npm install... mostly due to the bower substep...)
    • Then, edit uk.co.vedaconsulting.mosaico:composer.json to pull in the new JS build.
    • Then, do the release for uk.co.vedaconsulting.mosaico.