Bug 837836 - Lazy init nodes for templating from @KevinGrandon, remove BE dynamic load

Picks up @KevinGrandon's original lazy DOM approach, but updated for latest master. Also removes dynamic loader used for back end code since that non-worker code is now just a single file. Combining these changes for expediency and ease of uplift.

Related GELAM pull for the GELAM code:
https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/184