Add upgrade motivation banner

[knollengewaechs]

Steps to test:

• mock an outdated WK version, e.g. by mocking getBuildInfo(), see comment in maintenance_banner.tsx
• see banner, make sure it looks good (e.g. across different themes)
• test link
• click it away
• it shouldn't be shown again, even if you log out / close the browser. it should only be shown again after another 3 days
• it should be shown again in a private browser tab though.

TODOs:

• [x] fix styling of button, find out how to do theming well
• [x] fix banner when browser window is narrow
• [x] fix upcoming maintenance banner when user is logged out

Issues:

• fixes #7745

---

(Please delete unneeded items, merge only when none are left open)

• [x] Updated changelog
• [ ] Removed dev-only changes like prints and application.conf edits
• [ ] Considered common edge cases