Add preliminary support for SQLite

Miniflux can now be compiled with SQLite support. The next step are to actually
make it work.

- `CREATE TYPE … AS ENUM(…)` is PostgreSQL-specific, and is roughly equivalent
  to `CHECK(… in (…))`
- Specifying the index type is PostgreSQL-specific, so let's do it
  conditionally based on the used driver
- Replace `timestamp` with time` with `timestamptz`
- Don't use full-text search for sqlite for now
- `md5` isn't supported
- SQLite doesn't enforce types (yes.), so no need to alter tables to change
  them
- Various portability improvements