Release Prep for 2.4.8

Update CHANGELOG.txt
Update version.php

[skip ci]
Merge pull request #723 from jrjohnson/248changelog

Changelog and version update for 2.4.8
Fix incorrectly dropped procedure
Migration 29 in [405d7121f59fd1a48b9f1c81e71e727bb3858dc5] accidentally dropped the user_ids_from_cohort_and_master_group stored procedure.

This changes fixes migration 29 and also adds migration 31 to restore the procedure anywhere it was already dropped.
Merge pull request #724 from jrjohnson/subgroupsfix

Fix incorrectly dropped procedure
updated CHANGELOG.txt, ready for release 2.4.8
[skip ci]
updated CHANGELOG.txt, ready for release 2.4.8
[skip ci]
Merge pull request #726 from thecoolestguy/rb2.4.8

updated CHANGELOG.txt, ready for release 2.4.8