Core, Activity: change the Activity table's `primary_link` column from `varchar(255)` to `text`.

This change will avoid unintended truncation for long URLs. This can be a problem for links
built from slugs written in double-byte languages, or for letters that have been URL encoded.

We've tried previously to fix this, in r5966. Better luck this time. :)

Fixes #5890, props tharsheblows

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9168 cdf35c40-ae34-48e0-9cc9-0c9da1808c22