Mattermost-plugin-github: Use custom post type to display TODO list

[jwilander]

Currently, the TODO list that gets posted daily or by /github todo looks something like this:

This works ok but it's hard to read, is only minimally functional and I think we can add something that does way better. Plugins have a cool feature that lets you use custom post types to render your own React components in place of the regular post components. Leveraging this we can:

1. Display way more information about the pull requests/issues upfront in a much more readable way
2. Add more functionality, such as a button to open all the unread messages so users don't have to click through each one

I started working on this in the unreads-v2 branch with the following commit https://github.com/mattermost/mattermost-plugin-github/commit/54e8ecf1f797ad517f362e40424cea4b78eba371 but stopped for two reasons: 1) I ran out of time and 2) plugins could not use custom post types for ephemeral messages (which the /github todo command returns). As of the next Mattermost version 5.12 coming out soon, 2) is possible thanks to https://github.com/mattermost/mattermost-webapp/pull/2759. Unfortunately, 1) is still the case as I'm still a bit too busy to work on it.

If someone is interested in picking up where I left off I think this would be a really great feature that would take the GitHub plugin to the next level. It'll require a bit of everything from server side, front end, UX design and merging master into my branch.

Here's what it was looking like when I stopped:

[reflog]

@jwilander just an FYI, part of what this PR does wass already implemented in another PR merged recently: https://github.com/mattermost/mattermost-plugin-github/pull/111 . Not as pretty as your version, but it solves the problem of not knowing what the PRs/Issues are :)