Sitewide activity feed broken when activity component set as front page

[r-a-y]

Due to changes in r7437, the sitewide activity feed is broken when the activity component is set as the front page.

I've attached a quick patch to address this, but am unsure if this is the best way to address this,

[boonebgorges]

In 7471:

Don't canonical redirect page_on_front if bp_current_action() is populated

When looking at a BP component and bp_current_action() is empty, it means that
the item is (1) not a single item (like a group or member), and (2) not a feed
request. Therefore, it's a good way to infer whether a page is a directory
page, and ought to be considered for redirecting to directory page_on_front
settings.

This fixes the problem of /activity/feed/ being redirected to the front page
when the Activity directory was set to page_on_front.

Fixes #5211

Props r-a-y