Developer docs with db migration instructions

- Created a developer.rst file to put in developer docs.
- Added DB migration instructions.
Fix spelling mistake and redudant text per review
Merge pull request #556 from openspending/docs-db-migration

Developer docs with db migration instructions
Fix html/style for aggregate table embed

The bottom button bar was obscured. I can see no
reason why this widget would need to be 400px in
height.
Merge commit '3b26d' into experiment-split-store