feat: update expenses policy

[eseidel]

Summary

Replaces the GitLab-inherited spending guidelines with two questions, and adds a
travel section. Our old first guideline was "spend company money like it is your
own money", which GitLab has since dropped themselves, and which is vague enough
that two reasonable people read it very differently. We had nothing written
about travel at all.

• Spending guidelines are now "can you explain how this is in Shorebird's best
  interest?" and "would you be comfortable explaining it to the whole team?"
• The old numbered list moves down to Mechanics, since it was housekeeping
  rather than policy
• New travel section: economy by default, premium economy on a long overnight,
  upgrades on your own card, and raise exceptions before booking rather than
  after

[eseidel]

Folded into #220 (handbook refresh); the expenses.md diff is carried over unchanged.