RSS Feed: use bbpress functions for post and reply authors

[yoavf]

Currently, the RSS feed output from bbp_display_replies_feed_rss2 uses the_author() to output the display name for the topic and reply authors.

This prevents any filters added to bbpress core functions from being applied to the display names.
We should use bbp_author_display_name() had bbp_reply_author_display_name() instead.

[johnjamesjacoby]

In 7326:

Feeds: prefer filterable functions in bbp_display_replies_feed_rss2().

This change replaces function calls to the_author() with the bbPress equivalent for topics & replies.

This allows display names in feeds to be filtered in the same way they normally are in non-feed contexts.

Props yoavf.

In trunk, for 2.7.

Fixes #3543.

[johnjamesjacoby]

In 7327:

Feeds: prefer filterable functions in bbp_display_replies_feed_rss2().

This change replaces function calls to the_author() with the bbPress equivalent for topics & replies.

This allows display names in feeds to be filtered in the same way they normally are in non-feed contexts.

Props yoavf.

In branches/2.6, for 2.6.14.

Fixes #3543.