Add “Unit” to filter-by-type dropdown

[sdaitzman]

We should discuss whether we want to show units in the "filter by type" dropdown, and drop this story if not.

Spec

• Users can filter by "unit" type

In Scope

• Add "unit" to type dropdown

Not in scope

• Separate unit tab at library level
• Any unit subtypes

[bradenmacdonald]

@sdaitzman It seems weird to me to have both a tab and a filter option in the dropdown. I'd suggest one or the other.

[bradenmacdonald]

Though the dropdown does have the advantage that you could choose "units and subsections" for example, if they were both in the type filter, which you can't do with tabs. Not sure if we care.