Explicitly name template languages

Flow boards have the odd property of having hogan templates
and handlebars templates.
We have some hacky code in Mantle that deals with this currently,
but let's remove this by making things explicit

See: Iab1c1a9025bd3e1e3d3a2bb5625adbdb6f7fddd2
Change-Id: Icb9352e422dc105ac769bbe0db7e5525895204c7