Document database migrations and deployment

[jwhitlock]

For deploys, the database migrations are executed before updating the web services. This means that Tuesday's code (version N-1) needs to run with Wednesday's migrations (version N). This requires some developer and reviewer discipline, such as:

• Add a column or table in one deploy, and start using it in the next deploy
• Remove the code that uses a column one deploy, and drop the column in the next deploy

It would be good to add this advice to the development guides, and maybe the PR template.

┆Issue is synchronized with this Jira Task

[jwhitlock]

Long operations, like adding indexes, block requests. We should document some of the long operations we've seen during development, and advise on downtime procedures.