Add table comments to schema

[LeighMac]

Some of the tables in the 0000-00-00-schema.sql file have comments attached to them. ie the comment on the history table is "This table keeps track of ongoing changes in the database.". Comments can also be attached to columns (not currently done anywhere) such that they show up in show full columns from $tblname or show create table.

We should add comments to all the tables and consider adding some comments to columns so that new developers can see what the point of a table is. Perhaps base this on the descriptions that Christine wrote for Elodie?