Set hit_count on mapping create

At the moment, after you create a mapping, it will have a hit_count of zero,
even if there are hits.

Before we denormalised hit counts onto mappings this wasn't necessary as it
was calculated on the fly.
Merge pull request #336 from alphagov/set_hit_count_on_mapping_create

Set hit_count on mapping create
Convert config/migrations to PostgreSQL

* Remove structure.sql, add schema.rb, configure appropriately
* Remove mySQL-specific option values from migrations
* Remove fix_collation_woes, as is all mySQL-specific
* Remove mysql2 gem in favour of pg
Explicit TNA timestamp conversion

* mySQL used to implicitly convert this
* We aren't in Kansas any more
Use correct type for http_status

* mySQL would handle this implicitly
* Postgres doesn't, thank $DEITY
Fix list ordering in host/mapping_controller specs

* Used to work by coincidence in mySQL
* We're only interested in the set, not the order
Site#default_host depends on host insert order

* Make that explicit, we got away with it in mySQL
Get everything failing for a reason

* Raise RuntimeErrors with suggestions so we can see what
  class of problem breaks what in specs
* Gives us something to fix. Ignoring features right now,
  concentrating on specs
Postgres: fix organisation sites scope

* Digit boolean becomes boolean literal
* Can't alias the COUNT in the HAVING
* ONE, ah ha ha
* TWO, ah ha ha
Fix hits update

* Add upsert support that was missing from the spike
Fix the import of DailyHitTotal

* Split upsert into two queries
* Be specific about the behaviour the specs are testing
Fix sort by hits

* Postgres requires NULLS LAST on an ORDER DESC, apparently
* We can get away with the GROUP BY primary key trick for the rest
Fix HitsMappingsRelations import

* Straight mySQL -> postgreSQL swap
* INSERT IGNORE simulation and ANSI update style
Fix lack of type conversions

* A couple of dynamic columns used for counts on model scopes
  (usually SUM(something) as attr_name) were coming back as strings
  see https://groups.google.com/forum/#!topic/ruby-pg/egzlYNi2OKk)
* Lay a method on top of the attribute to do the conversion on the fly
* Better suggestions welcomed!
Fix mappings from host paths

* We really need the GROUP BY, and the way we tell Postgres that
  the path is effectively represented by the hash is to use MIN
  as the aggregate. We can no longer use #pluck as this will affect the
  SELECTed field, but we can get the same behaviour by simple #map.
Fix hit scope groupings

* Fall back on MIN for path_hash, mapping_id and host_id when grouping hits
Fix order-dependent test

* Can't rely on insertion order for entries, go by path
Postgres adapter backport

* From ActiveRecord 4.0
* Has path to allow hash of variables
* Allows increase in work_mem - vital for HashAggregate to be used instead of slower GroupAggregate
Widen hits and hits_staging path column

* Would fall over on odd too-long rows from hits
Turn off quoting

* CDN files occasionally have unmatched quotes in the path
* Postgres doesn't allow disabling quoting when treating a file
  as CSV just to skip the header, so pretend the quote is an
  unprintable (E'\b' == backspace)