Improve i18n in RSS feeds

[DJPaul]

In Activity's RSS feeds, such as /bp-activity/feeds/bp-activity-group-feed.php, the i18n is not good. There are hardcoded strings like this:

<title><?php bp_site_name() ?> | <?php echo $bp->groups->current_group->name ?> | <?php _e( 'Group Activity', 'buddypress' ) ?></title>

In this instance, gettext placeholders with argument swapping should be used. We should audit all the RSS.

[boonebgorges]

(In [5394]) Improves l18n in Activity RSS feeds. Fixes #3728