Ensure that date & relative date comparisons work in a consistent way across JSON, SQLite and MySQL

NQL date handling is in an unmanageable state.

We have to manage across the following variables:

• The format you use to insert into a DB (e.g. in fixtures) is different to what DBs return
• The formats you can use to do a comparison like > or < is not consistent between SQLite and MySQL
• The formats we return from the DB, and from our API endpoints can all be different
• JSON comparisons seem to work in some cases and not others, but really we should be using ISODate
• The mai use cases for dates are:

1. get helpers, where the real use case looks like this:
   {{#get "posts" filter="published_at:<='{{published_at}}'+id:-{{id}}" limit="3"}}
   this means it depends what comes out of the API as to what goes back in...
   ref: https://ghost.org/docs/themes/helpers/get/#filter
2. Prev & Next helpers which are builtins that do essentially the same thing, but they enforce that the right format
   ref: https://github.com/TryGhost/Ghost/blob/1cc38733ba6df8b8340420cff8fc7b643704c6ae/core/frontend/helpers/prev_post.js#L21
3. relative date queries like last_seen_at>now-1d
   ref: https://github.com/TryGhost/NQL/issues/26

We need to make sure that dates work consistently in these 3 cases.

Start by comparing the date handling in JSON, SQLite and MySQL, then read core/frontend/helpers/prev_post.js and the linked NQL issue #26. Trace the get helper, Prev and Next helpers, and relative-date queries such as last_seen_at>now-1d. Done means these three use cases compare dates consistently across the supported database formats and API values.