[Continued] Google Analytics import - #1753

This adds another entry to the JSON data returned via the main graph API
called `imported_plot`, which is similar to `plot` in form but will be
completed with previously imported data.  Currently it simply returns
the values from `plot` / 2. The data is rendered in the main graph in
red without fill, and without an indicator for the present. Rationale:
imported data will not continue to grow so there is no projection
forward, only backwards.

This changes "visitors" and in some places "sources" to atoms. This does
not change the behaviour of the functions - the tests all pass unchanged
following this commit. This is necessary as joining subqueries requires
that the keys in `select` statements be atoms and not strings.

This needs a bit more work on the google import side, as GA do not
report sources and utm sources as distinct things.

This extends the merge_imported function definition for sources to
utm_sources, utm_mediums and utm_campaigns too. This appears to be
working on the DB side but something is incomplete on the client side.

…ashboard view

This requires converting the `"visits"` and `"visit_duration"` metrics
to atoms so that they can be used in ecto subqueries.

Also changes a conditional to be a bit nicer

GA has only a "source" dimension and no "UTM source" dimension. Instead
it returns these combined. The logic herein to tease these apart is:

1. "(direct)" -> it's a direct source
2. if the source is a domain -> it's a source
3. "google" -> it's from adwords; let's make this a UTM source "adwords"
4. else -> just a UTM source