Improve linting git messages for compliance with gitmoji

[sampaccoud]

Feature Request

Related to https://github.com/openfun/richie/pull/196

Linting git commit messages for compliance with gitmoji could be improved:

• [x] download and cache gitmojis list of emojis directly from https://raw.githubusercontent.com/carloscuesta/gitmoji/master/src/data/gitmojis.json
• [ ] package the gitlint extra rule to pypi for use in other code bases