Add webhook without leaving mattermost

[manland]

As a user (before I dev this plugin) I didn't know how to add a project to listen to in github-plugin. I didn't read the readme... meaculpa. But I think to be more easy to use we should add webhook for user. Like that user just /gitlab subscribe owner/project and all the magic make the job.

Obviously for group, we need to fetch all projects and add it to every repos. If user create a new project after this init step we will miss it... maybe we can cron a synchronizer...

https://docs.gitlab.com/ce/api/projects.html#add-project-hook

[ericjaystevens]

I'll take this after #130

[manland]

@ericjaystevens I have started the Dev in #53 but I'm blocking because of missing feature in gitlab (API to add webhook for group). A contributor has started to work on it last week at gitlab... Wait and see!

[ericjaystevens]

@manland my understanding of your approach to this issue is to create hooks in the /gitlab subscribe command. This solution is great because it does auto-syncing and lets users not worry about webhooks.

An alternative approach we should consider is adding a sync switch the subscribe command that lets the users explicitly choose to use this approach. We would then also offer dedicated commands for people who want to manage project and group hooks manually.

So my proposal would look something like

/gitlab subscribe owner[/repo] [features] [syncWebhooks] - Subscribe the current channel to receive notifications about opened merge requests and issues for a group or repository
   - syncWebhooks - automatically creates project and team webhooks as needed to point to your Mattermost instance.

/gitlab webhook add owner[/repo] [features] - Adds a webhook pointing to your Mattermost URL.
/gitlab webhook remove owner[/repo] - Removes all webhooks for a group or project that points to your Mattermost URL
/gitlab webhook list owner[/repo] - lists all webhooks for a group or project.

[manland]

@ericjaystevens yes IMHO it's a good addition and with #36 it's a perfect fit for user to discover what is in sync and what not! I'm sorry I haven't time to do it (baby come 😅). If you want to continue #53 you are more than welcome!