Plugin Directory: Include slug in submission emails

[Ipstenu]

To mitigate the accidents of "Oh no! I didn't want THAT slug!" we should let people know what the slug's going to be when the plugin is submitted.

This patch tells them in two places:

1) The in-page notice of "Submission in!"

2) The email of "We got it!"

Both detail what the slug is and remind developers not to resubmit and just email us if there's a problem. This MAY result in a lot of weird emails about "WooCommerce! We need woo!" but that's better than after-approval complaints about bad slugs. I think. If I'm wrong, we'll try again.

[SergeyBiryukov]

2830.2.diff​ updates translator comments.

2830.3.diff​ is a combined patch with the changes from #2788, as they have some overlap. Whichever one gets committed first, should fix the other one as well :)

[SergeyBiryukov]

2830.4.diff​ is the same as 2830.3.diff​, but with raw email address converted to a mailto: link for better UX.

[SergeyBiryukov]

The new message looks like a wall of text: 2830.PNG​.

2830.5.diff​ splits it into several paragraphs to make it more readable: 2830.paragraphs.PNG​.

[tellyworth]

In 5561:

Plugin directory: include plugin slug in submission emails, and improve messaging generally. Props @SergeyBiryukov

Fixes #2830