Page layouts and containers

[rrhyne]

The new container system should be tested on various pages other than those addressed in the design process.

This effort should result in list of rules for the use of containers.

[rrhyne]

Involves the following efforts:

Code monitors:
https://github.com/sourcegraph/sourcegraph/issues/20933

Campaigns
https://github.com/sourcegraph/sourcegraph/issues/20935

Additionally, the extensibility team will handle the extensions page and insights will work with the insights page.

[rrhyne]

@sourcegraph/design we need to get this reviewed by tomorrow so we can move to ready for dev for next week.

[rrhyne]

PR: https://github.com/sourcegraph/sourcegraph/pull/21428