Implement saving and fetching watch functions.
Make saving work better.

Truncating the collection is ghetto, but all the data we want to keep
is in the POST. Doing this makes life simpler.
Add some doc blocks + tests.
Add some styling and behavior to the watch list.

You can only add one row at a time, but the page is fast, so I can live
with it for now.
Allow get() to get all metric data.
Make watch functions work on the run page.