Fix activity filter links

[johnjamesjacoby]

Because add_query_arg() uses $_SERVERREQUEST_URI directly and not from a passed link or variable, the previous method was removing arguments that would get immediately added again inside the loop.

This method rearranges the order a bit and relies on add_query_arg() to return the URI, and also removes the acpage from the filter links.

Previously acpage would still get passed, causing an unintended jump to filtered pages (i.e. - if status updates have 6 pages and groups have 2, changing filters causes a page-not-found.)

[johnjamesjacoby]

Fix bp_get_activity_filter_links pagination issue

[apeatling]

(In [1911]) Fixes #1050 props jjj