Add `collect=count` to allowed module query params

`count` is a valid argument to pass to the collect query parameter,
so Stagecraft should allow it to be part of the Module model.
Merge pull request #261 from alphagov/add-count-to-stagecraft-params

Add `collect=count` to allowed module query params
Example SQL to find dashboards with a module

SQL that can be used to find a list of dashboards containing a certain
module. Placed here for reference.
Merge pull request #262 from alphagov/sql-to-find-dashboards-containing-module

SQL to find dashboard with certain module