Add parent_domain_id links to databases
When adding new FTP and Shell users, they are linked to a specific site. Databases are currently only linked to a client.
If databases are also linked to a specific site, I will be able to fully implement #878 (closed), which is currently only possible for FTP and Shell users. Although there are rare cases where one database is used by multiple sites, the vast majority of sites will not share a database, so this should not cause a problem.
This would not be a "hard link" as such. Meaning, if a database is assigned to siteone.com, the client could still use the same database for any other site they own as they can now. This is merely an internal link to enable the [DOMAINID] prefix to be used with databases too.