Plugin Directory: Improve quality of automated emails and messages for submissions

[Ipstenu]

Attached is a patch to improve the default emails that are sent out following the submission of a plugin.

Changes:

• All errors now begin with "Error: short description" - this should make it more clear what's going on.
• Messages have been clarified based on feedback from developers
• Reminder not to resubmit
• Links to the guidelines and a reminder to subscribe to the blog added to the receipt email
• Clarified what happens AFTER a successful submission (we review etc)
• Added 'WordPress' to reserved slug - We now have the ability to change it afterwards for special cases!

[SergeyBiryukov]

Needs a refresh after [5471], but there's also some overlap with #2788 and #2830, so I guess I'd wait for those tickets to be committed rather than refresh several times.

Seeing simple text changes like this taking weeks to commit is not exactly encouraging, luckily there's plenty of other tickets to work on in the meantime :)

[SergeyBiryukov]

Refreshed after [5561], should be good to go.

[Otto42]

In 5574:

Plugins Directory: Text changes for emails and submission messages. props @SergeyBiryukov. Fixes #2700