When building get_views() links on Groups and Activity admin, build URL base from scratch

Previously, we were taking the current URL and removing query args by name.
Invariably, some query args would be forgotten about, leading to unpredictable
results. By building the base URL from scratch, we avoid any danglers.

Fixes #4901

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7175 cdf35c40-ae34-48e0-9cc9-0c9da1808c22