Ability to add a new column

[simonw]

This one doesn't require a complete rewrite of the table.

• [x] Add a column
• [x] Validate column name (cannot already be used)
• [x] Validate type
• [x] Show useful error messages

[simonw]

I want to redisplay the edit page with the error message shown there.

Thankfully I don't think I need to ensure the form is re-filled with the previous values, ala Django forms, because it's only a couple of fields.