Database versioning

[lladdy]

Don't deploy this to production until you've run the SQL at the bottom!

Feedback is appreciated. This PR is meant to allow for versioning of database changes so that semi-independent distributed development can take place.

What this PR changes:

• adds a very basic database version management utility and updates documentation
• adds a check in the website header.php to ensure it is running against the expected database version.

---

Before deploying to production, the database version needs to be set:

CREATE TABLE `schema_variables`
(
  `schema_version`   VARCHAR(255) not null
);
INSERT INTO `schema_variables` (`schema_version`) values ('1.0');