Update OAuth Error Page

Our error page for any failed or cancelled attempt at logging in with oauth (Github, FB, LinkedIn, etc) is currently unstyled, and not in the DemocracyLab page template:


The easiest way to get to this error page is:
- Go to DLab's login page (use staging, as it's set up for oauth and dev/prod-mirror are not)
- Select "Continue with LinkedIn" (or any OAuth provider, but for this example LI)
- Click "Cancel"
- Wait 5s for the redirect back to DLab.

To solve this issue, this error page should be:
- in the DemocracyLab page template (i.e. showing our header, footer, etc)
- If this requires a new page (it might) creating and routing to a new front end section, see issue #493 for an example of page creation, and/or ask on slack until we get that properly documented.
- We may be able to reuse the existing [ErrorController](https://github.com/DemocracyLab/CivicTechExchange/blob/12ed1b4cd359eadc99c577edaa5b79e079aef90f/common/components/controllers/ErrorController.jsx), and that'd be my preference if possible.
- remove the Menu section of the page content, those links are broken
- the remaining page content can otherwise stay unchanged for now -- we'll do a separate ticket for the error text since I'll ask one of our UX Writers to handle it.

More broadly,
- find out where this page content is coming from
- find out how to display one of our front end sections instead, so we can make it clear to users they're on DemocracyLab and seeing a DemocracyLab error

Pull Request #386 and may provide useful context for this issue.