Stats: Make header on traffic page sticky

[annacmc]

Related to https://github.com/Automattic/red-team/issues/217

Proposed Changes

• Makes the header on the traffic page sticky

Why are these changes being made?

• Part of the Date filtering for the traffic page project (header refresh section)

Testing Instructions

• Open the Calypso Live link.
• Visit the Stats → Traffic page and confirm it loads normally.
• Manually enable the feature flag: ?flags=stats/new-date-filtering
• Scroll the page and confirm the header sticks to the top of the page.
• Keep an eye out for unintended layout changes.

Design Image	Screenshot from branch

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-125533

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-125533&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-125533&env=a8c-for-agencies

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• notifications
• odyssey-stats

To test WordPress.com changes, run install-plugin.sh $pluginSlug update/stats-sticky-header-component on your sandbox.

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~1031 bytes added 📈 [gzipped])

name   parsed_size           gzip_size
stats      +3987 B  (+0.4%)    +1031 B  (+0.3%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[kangzj]

Let's ship first and iteration cuz it looks mostly good to me!