Add Subscribe to plugin commits via email widget/functionality

[obenland]

Any user can opt to subscribe to a plugins commits via email.

To implement this we'll need

• The ability to subscribe/unsubscribe to a plugins commits, probably as a widget or shortcode depending on theme design.
• An authenticated REST API endpoint to retrieve the email addresses of users who should be notified of a plugin commit. All plugin committers are automatically (forcibly?) subscribed.

[dd32]

In 3510:

Plugin Directory: Allow a user to subscribe/unsubscribe from plugin commit emails.

Note: Plugin committers are automatically subscribed, and cannot unsubscribe.

See #1597

[dd32]

In 3516:

Plugin Directory: Add a method to retrieve all the users who receive plugin commits.

For now, this also includes the bbPress subscribers until the final switch over.

See #1597

[dd32]

The API now uses the new method added in r3516

[samuelsidler]

Milestone Plugin Directory v3 - M5 deleted