Support the GitLab plugin for mattermost

Problem to solve

There is a community plugin for Mattermost available that adds several notification related features to Mattermost regarding your GitLab projects. : https://github.com/manland/mattermost-plugin-gitlab

The plugins is a fork of Mattermost's own GitHub plugin: https://github.com/mattermost/mattermost-plugin-github

Intended users

Anyone using both GitLab and Mattermost

Further details

This was brought up today during a meeting with @jasonblais from Mattermost. They are looking at officially supporting the plugin, and would like us to consider shipping it with Omnibus.

Proposal

We should evaluate whether we want to support the plugin, and which steps to take:

• Referencing it in our Mattermost integration documentation
• Shipping it in Omnibus so it is available with your Mattermost install (omnibus-gitlab!3536 (merged))
• Include support for it in our Mattermost integration to simplify the GitLab side of the setting it up: https://github.com/manland/mattermost-plugin-gitlab#installation