Add param to bp_get_activity_action() so timestamp can be skipped

[DJPaul]

bp_get_activity_action() currently returns a timestamp suffixed to the activity->action text. There's no way of preventing the timestamp being suffixed to the text, which limits how the function can be used in theme templates.

Add a parameter to allow the timestamp to be skipped.

[djpaul]

(In [6489]) Add parameter to bp_get_activity_action() to prevent the timestamp being appended to the action text. Fixes #4648

This was limiting how the function can be used in theme templates; some themes may want to show the timestamp somewhere else.

[r-a-y]

In 12564:

Activity: Minor PHPDoc audit for bp_get_activity_action() function.

See #8234, #4648.