Linkable Blog index filter needed

[vossisboss]

Every blog in our new design has a Category assigned to it so that blog readers can filter the content according to the types of articles that interest them.

As a user, I should be able to:

1. Filter the blogs according to category
2. Share a link for the filtered page that contains all of the blogs in the filtered category

For example, if I was a user interested in reading all of the "Adventures in Wagtail" posts, I would want to be able to filter the blog so that only those articles were displayed and then share a link for that filtered page with a colleague of mine.

[zerolab]

Work in progress PRs:

• #259
• #271