Use model to establish sort params instead of DOM

This means that tables which have their sort params and directions specified in config (such as those on site activity dashboards) won't be overwritten due to the the lack of DOM properties.
Handle the case where no sort order is defined on model

For graph tables, no sort order is defined by default, so just leave the table as is.
Ensure the services don't overlap the departments.
Merge pull request #709 from alphagov/bugfix/table-sorting

Use model to establish sort params instead of DOM state
Merge pull request #711 from alphagov/bugfix/services-page

Ensure the services don't overlap the departments.
Updated instructions for deploying an app to Heroku
Merge pull request #713 from alphagov/documentation/heroku-readme

Updated instructions for deploying an app to Heroku
Update site activity dashboard taglines
Update site activity content dashboard straplines

"Content dashboard" => "Dashboard"
Updated titles for site activity content dashboards
Merge pull request #714 from alphagov/feature/update-site-activity-copy

Feature/update site activity copy
Update cheapseats pointer

There were some assumptions being made about date formats which have been exposed now there's a difference between "long" month names (i.e. "August"), and short month names (i.e. "Aug").
Merge pull request #715 from alphagov/bugfix/date-formatting

Update cheapseats pointer
Remove timeshifts from site-activity visitors module

It's been decided that using the date pickers is a preferable soution to displaying timeshifted data.
Add "delta" table processor

Returns the change compared with the previous period.

Only for non-date-indexed collections for now i.e. where the x-axis key is not a timestamp.

Simplified the code to test if a key is Date-y to match logic used when parsing data, and re-used function for both.
Add delta columns to help usage tables
Expanded and renamed test for parsing collections

The name of the test was terrible, but covered the basic cases for non-date-indexed datasets.

Expand out the tests so that they cover the new cases where datasets with a duration of > 1 copy their value to the top level, but leave the values array in place.
Merge pull request #716 from alphagov/bugfix/timeshift-labels

Remove timeshifts from site-activity visitors module
Defend against undefined axes options in collections

Some modules do not need to have any axes defined e.g. KPI modules, and so we can't assume that these will exist when parsing backdrop responses.
Update feedback link to make it more meaningful.