Add feeds for plugin support forum topic replies

[kasparsd]

The ability to receive notifications for new replies in the WP.org forum threads would enable plugin authors and maintainers to review and reply to those threads in a timely manner without having to subscribe to email notifications for each thread.

Currently we implement the following custom feeds on WP.org plugin support forums in addition to the standard bbPress feeds via the support-forums plugin:

• Plugin ontributor feed ​https://wordpress.org/support/view/plugin-contributor/sergeybiryukov/feed/ (defined here)
• Plugin commiter feed ​https://wordpress.org/support/view/plugin-committer/sergeybiryukov/feed/ (defined here)

Those feeds currently list only the forum topics created for the plugins associated with that user. Could we extend those feeds to also include the topic replies?

Or create a support "firehose" feed for individual plugins that include both the topic and the replies? This sounds like a better general solution since anyone could access those.

Somewhat related to #2071.

[netweb]

Related: #2204