Nothing flashy today; I was up all night trying to download photos taken 
by a robot lowered onto Mars by a skycrane.

Some work on alerts. Added an alert when a file transfer succeeds or fails.
Improved the alert combining code so it handles those alerts, and
simplified it a lot, and made it more efficient.

Also made the text of action alerts change from present to past tense when
the action finishes. To support that I wrote a fun data type, a `TenseString`
that can be rendered in either tense.
