Align one line of code in snapshot model
Add counter caches to sweeps

I wanted to display a short text explaining the status of a sweep.
Instead of making expensive count queries for every sweep, I decided to
save some counter caches instead. This might be a tiny bit YAGNI at this
point, but I'm pretty sure we'll find them useful moving forward.

The only user-facing outcome of this commit is that the sweeps#index
page will have a new column named "Status". This column will display
with a few words the most important information about the snapshots in a
sweep.

If you need to backfill old sweeps, run this:

```bash
bundle exec rails runner "Sweep.all.each {|sweep| sweep.update_counters! }"
```