Add properties file build

Adds a grunt task called `content` to build `conf/content.properties` file from the YAML files in `conf/digital-marketplace-ssp-content`.

When the task is run `conf/content.properties` is stored as an artefact ignored by Git.

This could be done by running the following:

`node run_grunt.js content`

Note: the task uses new NPM modules so you'll have to run `npm install` before running it.