Code quality improvements to BP_Group_Membership_Requests_Template

- Refactor constructor to accept params as an associative array
- Don't use extract() in constructor
- Docblock

See #5440

Props dcavins

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8066 cdf35c40-ae34-48e0-9cc9-0c9da1808c22