Add three-column grid widget

[benlk]

Changes

This pull request makes the following changes:

• Removes Sphinx from Gruntfile. Since we're not compiling docs here, it's unnecessary for the grunt watch task
• Adds a post widget with three columns, for #33
  • based on Largo Recent Posts, but with code standards applied for https://github.com/INN/largo/issues/1807
  • Flexbox styles so it's responsive, and can be used places that aren't this page
  • adds drop-down for choosing Prominence terms that apply to the query
  • [ ] moves the "more" link to the top on desktop

Why

For #33

Testing/Questions

Features that this PR affects:

• widgets, series landing page

Questions that need to be answered before merging:

• [x] Is this PR targeting the correct branch in this repository?
• [x] what URL do we want to use for the "Featured Stories" view-all link?

Steps to test this PR:

1. Add a "Three-Column Recent Posts" widget to the sidebar for a series landing page
2. Set it to:
   • number: 6
   • avoid duplicates: yes
   • thumbnail: small, left
   • excerpt: no
   • byline: no
   • top term: no
   • limit to author: none
   • limit to prominences: Featured in Series (for the "Featured Stories" one)
   • limit to category: none
   • limit to tags: none
   • limit by custom taxonomy: series, borderlands-project
   • link text: View All
   • URL: /?prominence=series-featured&series=borderlands-project

[benlk]

This link format works for the Series Featured "view all" link: /?prominence=series-featured&series=borderlands-project

[benlk]

Updating screenshot with header button alignment from #75.

Note that this layout is as it is because there's only 5 posts that fit the criteria on my local db.