Previous code accepted the `scheduledAt` value as a String, and kept it
as a String (including when serialising as part of a draft). Then it was
converted to an actual Date for display.
Refactor to keep it as a Date for as long as possible. Moshi decodes
Dates correctly over the network, and the database is configured to
serialise Dates as Longs.
This necessitates two migration steps to preserve any existing
`scheduledAt` values for drafts. The first step adds a new column to
store the date as a Long and copies over existing data. The second step
replaces the old column with the new column.
Nik Clayton