Update mattermost to 3.7.1

mattermost/.gitignore

mattermost/.travis.yml

mattermost/MAINTENANCE.md

mattermost/README.md

mattermost/app/Dockerfile

mattermost/app/Dockerfile-enterprise

mattermost/app/docker-entry.sh

mattermost/db/Dockerfile

mattermost/db/make_db.sh

mattermost/db/setup-wale.sh

mattermost/docker-compose-v1.yml

mattermost/docker-compose.yml

mattermost/web/Dockerfile

mattermost/web/docker-entry.sh

mattermost/web/mattermost

mattermost/web/mattermost-ssl