Update templating to work with Hawthorn edX platform release

Switch xBlock's templating from standard the django get_template () to the xblockutils loader.render_template().

Move cached data from body to seq_content

Previously, some future update or rogue script could easily tamper with
our cached data attributes on the body element. We moved these data
attributes to be slightly less widely accessible.

Arose from our convo w/ free text response: Stanford-Online/xblock-free-text-response#21
Trello: https://trello.com/c/QB46SgTN/954-move-cached-data-from-body-element-to-seqcontent-for-submit-and-compare-xblcok

Dev: @potsui