Fix is_replicator_db_fun to allow usernames

This updates couch_replicator:is_replicator_db_fun to allow for
usernames as valid replicator db names. This issue presented itself in
multi-homed replications where the username was a numeric username.

This change introduces two subtle issues, 1) this will no longer fully
validate on dbnames, as dbnames are not allowed to start with a number,
and 2) this will not properly validate on usernames as usernames are not
allowed to have all the special characters available to dbnames.

BugzID: 23315