This sets the default feature to sqlite, so that it will correctly
compile "out of the box". Users can still choose different backend by
running:
```
cargo build --no-default-features --features <backend>
```
This includes migrations as well as Dockerfile's for amd64.
The biggest change is that replace_into isn't supported by Diesel for the
PostgreSQL backend, instead requiring the use of on_conflict. This
unfortunately requires a branch for save() on all of the models currently
using replace_into.