Allow plugins to override ephemeral posts

[esethna]

If you're interested please comment here and come join our "Contributors" community channel on our daily build server, where you can discuss questions with community members and the Mattermost core team. For technical advice or questions, please join our "Developers" community channel.

New contributors please see our Developer's Guide.

---

Notes: Jira ticket

Mattermost supports plugins that offer powerful features for extending and deeply integrating with both the server and web/desktop apps: https://developers.mattermost.com/extend/plugins/

Currently, plugins can override a regular message posted to a channel using registerPostTypeComponent - this allows a webapp plugin to render a custom body for posts with a specific type prefixed with 'custom_'.

The proposed change is to make this component work with ephemeral posts as well, which are only visible to the user.

A sample use case is for our Mattermost GitHub plugin, where "/github todo" returns an ephemeral message with items to do in GitHub. We would like to replace the resulting message with a custom UI instead of the hard to read Markdown list it is today, as attached below.

If you have any questions about the proposed change, ask jasonblais or hanzei on this GitHub issue, or message them on https://community.mattermost.com.

[jasonblais]

@hanzei can you help add the appropriate labels for this ticket?

[hanzei]

@jasonblais Done

[scottleedavis]

I'd like to try this one @hanzei @jasonblais :)

[hanzei]

Greet! 🚀 Thanks for all your work, Scott!

[hanzei]

Hey @scottleedavis,

I'm wondering if you would be interested in submitting a PR to the demo plugin that makes use of this new feature? Would be cool to show this new possibility in the plugin.

[scottleedavis]

Sounds great @hanzei , will do. 👍