Custom taxonomy term page

@sergii-tkachenko Can you review?

This is a work in progress, getting a custom tpl ready for #3159.  It starts on a custom Taxonomy Term page callback, because the core taxonomy's `taxonomy_term_page` function loads all nodes associated with a term, which we don't want.  

We only want `campaign` and `campaign_group` nodes to display, hencing adding the new `campaigns_by_term` view to `dosomething_campaign` module.

Coming next:
* Template file for taxonomy term
* Theme function for the Campaign Group and Taxonomy term to share the same Campaign gallery markup