Geoff Lawler authored Aug 29, 2019 and Ryan Goodfellow committed Aug 29, 2019

Experiment source can now be pushed to an experiment along with
the experiment XIR. The source is treated as an opaque string and
is stored in the database along side the xir.

When the experiment data is pulled, the source is provided
as well as the xir.

Note: the indentifier for the xir in the code was `src`. This
has been changed to `xir` to make clear that it is xir and not
anything else. The "source" is identified as `src`.

The xir is still indexed by /xp/[hash]/model in the database
though for backwards compatibility.

NOTE: This is a API breaking change! Both the `XpPush` and
`XpSource` structures in the Merge API now refer to the
xir as `xir` (instead of `src`) and the optional source
as `src`.