Add GitHub bot for PR/issue mentions

[ocean90]

There is currently a Trac bot which posts a comment on a ticket if it was mentioned on Slack.

It would be nice if we can have one for GitHub too. Projects like GlotPress, REST API, Dashicons or Twenty Sixteen would benefit of it.

[GaryJ]

Just to add - pulling mentioned Issues / created PRs ​Gutenberg into #core-editor and ​WPCS into #core-coding-standards would also likely be useful.

[jbpaul17]

Note that this ​came up today in #editor during a Gutenberg bug scrub. Any chance of looking back into having Slack mentions of Issues (and maybe even PRs) automatically referenced in the respective GitHub Issue (or PR) to ensure all conversation is appropriately captured in GitHub?

[ocean90]

The main reason for the maybelater was that GitHub has also sent email notifications for comments by bots. Not sure if that has changed.

[jbpaul17]

@ocean90 I'm guessing the maybelater is due to the fact that Slack references included in Trac tickets does NOT generate a Trac email notification? I'd counter that it may not be a horrible thing if GitHub generates an email notification for Slack references. Though I can see why people would want to mimic the same functionality as the Trac integration.