batches: fix timeline modal overflow

Created by: courier-new

Closes https://github.com/sourcegraph/sourcegraph/issues/36184.

This PR does a couple things to make the timeline modal with everything expanded overflow better:

• Moves the modal position from top third (default) to center
• Caps the max height of the modal to just under the visible screen height
• Wraps the actual timeline modal contents in overflow: auto so that it's scrollable

This also fixes the Total Time duration wrapping weirdly in the workspace details.

https://user-images.githubusercontent.com/8942601/172937615-89a578bc-74d1-4c39-93ee-a75134744902.mov

Test plan

Manually tested across different screen sizes (sorry, mobile users) with different states of expanded timeline contents.

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.