Add terms filtering to items API

As Dashboard has WidgetInstance objects, TabbedPage has TabInstance

The template is a temporary one for now

Add a test to ensure taxonomy slugs cannot contain colons. This will prevent regression and hard to find bugs in the future.

Allow filtering list of items by taxonomy term. The filter is build as <taxonomy_slug>:<term name>, so for example the following url filters on two terms (AND filtering): http://example.com/api/items?terms=taxonomy-one:a%20term&terms=taxonomy-two:another%20term . Note that taxonomy slugs cannot include colons, so no escaping is needed.

Rumor columns

Item types

Ensure we use the right taxonomy for the question category. The taxonomy in use is still hard-coded, as that is something we will look at later - this commit was only intended to restore functionality to the existing level.

added higher z-index to featured image to make sit on top of sidebar …

For RapidPro import - new column 'network provider' on item

There are two new item types 'question' and 'rumor'.
Item types for GeoPoll data are 'question'.
Item types for RapidPro data are 'rumor'