Mattermost-plugin-github: Add PR Title in Notifications DMs

[mattermod]

The Github plugin sends DMs for Github notifications related to PRs, i.e. when your PR is approved or when you are requested a review on a PR. The current format of the DM is:

These type of DMs should be improve to show the PR title as the text of the link to the PR. You can check out the text templates defined for the plugin’s webhooks here

---

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.

JIRA: https://mattermost.atlassian.net/browse/MM-18878

[iwataka]

Hi, I'm interested in Mattermost development.
Can I work on this issue ? (this would be the first time to contribute to Mattermost)

I have two questions for this issue.

1. I guess the message should be like the below, is it right ?

   [\[mattermost-plugin-github\]](https://github.com/mattermost/mattermost-plugin-github) [panda](https://github.com/panda) commented on your pull request [#42 Leverage git-get-head](https://github.com/mattermost/mattermost-plugin-github/pull/42)
   

2. Should I add not only PR title but also issue title ?

[amyblais]

@marianunez ^

[marianunez]

Welcome @iwataka! Thanks for taking this one on.

1. That looks about right, I think we just won't need to include the repo name at the beginning of the message. So, in your example it would look something like:

   panda commented on your pull request #42 Leverage git-get-head

2. Yes! That makes sense to be consistent, I missed to include the issues as well.

[iwataka]

Thanks for your help, @amyblais and @marianunez !

I will create PR to mattermost-plugin-github in a few days.

[iwataka]

I created a PR, @amyblais @marianunez .
But I sent a form about contribution license agreement just before, so please wait until my form is processed.

[lindalumitchell]

Test @prapti