Add meta description tags

This isn't ready to merge - I'm going through and adding meta description tags for pages and wanted to open this up to get feedback on the wording.

Closes #736 when it's done.
Yes, it's now set in the common header partial: https://github.com/openaustralia/publicwhip/pull/774/files#diff-c0905b0a131d69e99f0f18c7a1a0dda3R1