smarter markdown checks in i18n import

fixes CNVS-15576 frd

this way we don't treat the "# Assignments" as a heading during our import
checks

test plan:
0. specs
1. import the latest transifex stuff
2. it should not complain about "# Assignments"

Change-Id: Ic572c88aa4e6d7151d4c9647a6ac94b8af3612e0
Reviewed-on: https://gerrit.instructure.com/43422
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jennifer Stern <jstern@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>