Add Wagtail inventory plugin

[alanmoo]

https://github.com/cfpb/wagtail-inventory

This will enable us to search the site for where specific Streamfield components are used. Highly useful as we build out our component system.

I haven't tested this yet, so we should confirm it's an actually useful thing to add before opening a PR.

[alanmoo]

Looks like it needs to be updated to support Wagtail 2. I filed a ticket

[Pomax]

landed, we forgot to tie the re-PR to this issue.