Cache web cover width and height

[dfabulich]

Fixes #474

I manually tested this by:

• loading test data and switching to comp phase 5
• I added logging to _read_web_cover_geometry to track hits and misses
• I navigated to http://localhost:8080/ballot
  • In my Docker logs, I saw that the cache was missed
  • In Docker shell, I saw that entries/100/web_cover/geometry.txt had just been created
• I refreshed the ballot
  • In my Docker logs, I saw that the cache was hit
• I logged in as nobody@example.com and navigated to http://localhost:8080/entry/100/update, and uploaded a replacement image.
  • In Docker shell, I saw that entries/100/web_cover/geometry.txt had just been updated
• I refreshed the ballot
  • In my Docker logs, I saw that the cache was hit