Refactored activity_snippets

Needs to play nice with @virginiacc's PR though, so we should wait until her PR is merged before merging this.
Could we make the activity_types an optional argument that defaults to the full list? Maybe:

{% macro activity_snippets(tag, activity_types=['posts', 'op-ed', 'press release', 'speech', 'testimony']) %}

Then you'd only need to provide a value if you wanted a subset.