Membership dashboard is wrong (CRM-12500)

Stoob noticed that the counts of new and renewing members were very wrong in the membership dashboard.  The issue is that a "new" membership was one with the start date and join date in the given period, and a "renewing" membership was one with the start date in the period and an earlier join date.  Since implementing membership upgrades, this has been incorrect.

For now, this fix relies on the "Membership Signup" and "Membership Renewal" activities.  civicrm_membership_log may be a good long-run solution, but it doesn't distinguish between a renewal increasing the end date versus an upgrade increasing the end date.  Upgrading from a 1-year membership to a 2-year membership isn't a renewal.

The links to the membership search won't generally return accurate results, so they have been either disabled or replaced by links to an activity search.  (Activity search is able to look at new or renewing memberships in a time period, but not filter by membership type.)

There should be an effort to get this working correctly in 4.4; this is a stopgap to at least make sure we're displaying accurate information.