Fix bad interaction between pluggable InputTypes and Microsites

Pluggable InputTypes use edxmako.paths.add_lookup to inject new mako template
directories. Those were getting clobbered by microsites
(and by theming), which were completely rebuilding the mako template lookup
list from scratch.

This commit switches those two features to use the same add_lookup function as
pluggable InputTypes.

[LMS-2489]