ui: add optional white background to PageConfig

[absterr08]

Previously, the PageConfig component had a fixed grey
background to match the page background, since it is a sticky
component that needs a background color to cover content that
scrolls beneath it. However, the background color is changing
to white in CockroachCloud, so the background color needs to
be able to match the new background. Until we apply the layout
changes to db-console, this change allows us to specify a
different background color in CockroachCloud.

This change uses CockroachCloudContext to determine whether or
not the PageConfig background color should be white.

Without white background

With white background

Release note: None

Release justification: ui change for cluster-ui components used
in cockroach cloud

[cockroach-teamcity]

This change is 

[maryliag]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @xinhaoz)

[absterr08]

bors r+

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[absterr08]

TFTR!

[absterr08]

blathers backport 22.2

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from d9632c0 to blathers/backport-release-22.2-87313: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.2 failed. See errors above.

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