Experiment with small caps as a font feature

[gnprice]

In some places in the web UI, we've started to use small caps. It would be good to demonstrate doing so in Flutter.

Flutter has extensive support for font variations. See FontFeature, and more generally TextStyle. So it should be straightforward to demonstrate this, given a font that has small caps in the first place.

In particular Source Sans fonts have several features including small caps (smcp) and slashed zero (zero). So we can do this once we're bundling such a font: #64.

Other related issue:

• A different axis of fancy font features: Experiment with a non-centenary font weight #65

[terpimost]

The place where we about to use small caps is dates:

Not critical but very nice to have

[gnprice]

This is now effectively part of #157. Closing in favor of tracking there.