Taxonomy Term partners

* Adds `field_partners` field collection to Cause, Issue, and Action Type vocabularies

* Refactors `dosomething_taxonomy_get_partners_data` to expect the `field_partners` field array, instead of a node, allowing a taxonomy term to call the function as well (Refs #3162)

* Adds `paraneue_dosomething_get_sponsor_logos` helper function to return the `sponsor_logos` template from given `$partners_data` array (as returned by `dosomething_taxonomy_get_partners_data`)

Some of this PR works towards moving all Partners preprocessing to the  `dosomething_taxonomy_get_partners_data`, which is used by Campaigns and now Taxonomy Terms in order to start DRY.  ~~An ongoing todo has been to refactor the `node--campaign-group.tpl.php` to use the same `sponsor_logos` template.~~

^^^
**UPDATE** Refactors `campaign_group` to also use the same `sponsor_logos` theme function.
Should be string, e.g. `''`.