Admin: Front-end for Flow pages

[machikoyasuda]

closes #2634

WIP

General idea

• Extract flow-base as a template for the whole flow
• Move all pages into that, including error page
• Use default Bootstrap color class namings as much as possible
• Then, after the page is all laid out and all pages in flow are aligned, then edit the Bootstrap CSS Variables to make the secondary, secondary-subtle colors be the right grays.
• Ensure the other pages are not affected

Goals: Do not create any new classes and try to do default overrides as much as possible. Do not declare styles on existing classes.

Notes

• Re-doing the header to the new design is out of scope for this ticket.
• Re-doing the button CSS is also out of scope for this ticket.

[github-actions]

Coverage report

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