Stars are keyed off username, not userid, so don't survive user renames.

to reproduce:

1. login as "bob", and star some stuff
2. rename "bob" to "databasenormalisation101"
3. relogin as "databasenormalisation101" and notice that there are no starred items

to fix this:

1. normalise the CRU_BASE_STAR_MODEL table so that stars are keyed off user ids instead of user names
2. write a database upgrade script. Will need to cope with the possibility that there are usernames in the CRU_BASE_STAR_MODEL table that don't exist in the CRU_USER table. Can delete row in star table if username no longer exists in CRU_USER table?