Extend DB schema to record contacts

- Add contacts table to record contact details
- Extend requests table to record contact prefs
- Update ERD

This is the first stage of implementing openrightsgroup/cmp-issues#6. All done using mysql-workbench. I have included a .mwb file with an updated ERD for convenience.

The [structure.sql file in the Blocking-Middleware repository](https://github.com/openrightsgroup/Blocking-Middleware/blob/master/mysql/structure.sql) is out of date. Once everyone is happy with these database changes I propose we should merge this version of structure.sql over to that repository to make it current.

I wasn't sure whether I should change the requests table ID to INT(11) from INT(10) now that I've made it UNSIGNED rather than SIGNED (which was necessary to allow the FK constraint to the new contacts table).

Peer review please? @dantheta, @NetworksAreMadeOfString