Membership dash counts fixed but no good search

This fixes the membership dashboard counts for CRM-12500.  However, in doing so, it removes all links to searches.  The rationale is that no existing search accurately displays the number of new or renewing members in a given period.  The membership search shows the original join date and the start date of the contiguous membership period.  The activity search should theoretically work for displaying membership signup and membership renewal activities, but it isn't working for that, even in a plain vanilla search (this may be a separate problem).

Long run, there should be a search on the membership log table, presumably from the membership search.  In the meantime, I think it's preferable to give accurate counts without links than to give inaccurate counts with links to inaccurate search results.