MM-15324 Allow plugins override ephemeral posts

[scottleedavis]

Summary

This PR enables ephemeral posts with a post.prop.type to override a post by a webapp plugin.

e.g. keeps the system_ephemeral post.type, but allows post.props.type to override

		p.API.SendEphemeralPost(args.UserId, &model.Post{
			UserId:    p.botId,
			ChannelId: args.ChannelId,
			Message:   "Bot ephemeral link",
			Props: model.StringInterface{
				"type": "system_ephemeral_test_plugin",
			},
		})

This has been validated by using /test-ephemeral-post-override from the test bot plugin

Ticket Link

Fixes mattermost/mattermost#10797

Notes

I did not add a test to post_message_view_test.jsx as I wasn't sure if it was needed and I am unfamiliar with snapshot testing. (happy to do it if requested).
I did not add a test for post_body.jsx as no tests currently exist for the component.

[hanzei]

@cpoile Gently reminder about this PR. Would be great if we can get this in v5.12.

[cpoile]

Thanks @scottleedavis, this is really nice.
Just making sure I understand what's happening here: you have to add your own props to an ephemeral post, and that's how it'll pick up that this is post that 'hasPlugin'?

[scottleedavis]

Thanks @cpoile ! That is correct. I chose this route over change the post type to have a prefix 'system_ephemeral_<custom_name>', as a lot of things would need to change, and I wanted to preserve the behavior with as little change as possible.

[jasonblais]

@hanzei @scottleedavis does this need any documentation at https://developers.mattermost.com/extend/plugins/?

[scottleedavis]

@jasonblais I think it would be a good idea. Otherwise it isn't straight forward to use props to solve the issue. I haven't done documentation updates yet, happy to do so if requested. :)

[jasonblais]

@scottleedavis It would be great if you had time to help with the doc! :)

[aaronrothschild]

Documented via: #3024 . Thanks Scott!