Come up with migration strategy for schema changes

[patrickhulce]

We need a way of dealing with changes to table schemas between version of LHCI, no promises made until initial published version but migration should be smooth after that

Ideas:

• Use https://github.com/sequelize/umzug for table schema changes
• Run migrations on every startup
• Store a version tag for each precomputed stat record
• On each query for computed stats, check for current version tag and compute on the fly

Questions:

• How do we handle future alternate storage mechanisms like bigquery?
• How do we handle potentially expensive API queries the first time after an update? Should we precompute the stat records?