add linter to prevent LFS files from being checked in

[indirectlylit]

Observed behavior

on at least three occasions now we've had binary files inadvertently checked in:

• https://github.com/learningequality/kolibri/pull/7092
• https://github.com/learningequality/kolibri/pull/6802
• https://github.com/learningequality/kolibri/pull/6467

Expected behavior

travis should block merges of PRs that have these files checked in

User-facing consequences

frustration for devs

Errors and logs

Previous HEAD position was e404c44740 Merge pull request #7082 from learningequality/release-v0.12.x
HEAD is now at 6979d0610c Merge pull request #7085 from DXCanas/dockerize-integration-worksheet
Encountered 54 file(s) that should have been pointers, but weren't:
	kolibri/core/static/assets/fonts/noto-full.NotoSans.400.woff
	kolibri/core/static/assets/fonts/noto-full.NotoSans.700.woff
	kolibri/core/static/assets/fonts/noto-full.NotoSansArabic.400.woff
	kolibri/core/static/assets/fonts/noto-full.NotoSansArabic.700.woff
	kolibri/core/static/assets/fonts/noto-full.NotoSansArmenian.400.woff
	kolibri/core/static/assets/fonts/noto-full.NotoSansArmenian.700.woff
	kolibri/core/static/assets/fonts/noto-full.NotoSansBengali.400.woff
...

Context

0.14.0 b3

[rtibbles]

This should be ameliorated by running pre-commit checks as part of CI, but the default large file size there of 500KB would not catch all of these files (the smallest of which seems to be about 17KB).

[marcellamaki]

fixed in #13947