Clean up existing OSC work

[fureigh]

At a high level, this PR represents the following:

1. Rebased on this repo's master branch.
2. Pulled in most elements of cf-ex-drupal8.
3. Restored OSC-specific theme and some other elements of content and configuration from the add-initial-header-footer branch, where work was happening most recently.

Sharing as a WIP to more effectively enable @pburkholder to run with the remainder.

Recommended next steps:

• [ ] Revisit core.extension.yml; it's likely not entirely accurate at this point.
• [ ] Fix deployment; ditto.

Several of the remaining visual elements can be quickly reproduced through the GUI, thanks to the USWDS Paragraphs module (which is included here).

[fureigh]

@tbaxter-18f Cool to merge this as-is, for the sake of clarity, and break out the remaining issues?

[tbaxter-18f]

Yes. let's do it!