Standardize gallery URLs

[atomtay]

We currently have two URL "forms" for our data visualizations: first, users go to /gallery and pick a year (no change to the URL), and then click a month. Assuming that the month's visualization isn't a PDF, the user goes to /calendar/:year/:month. I think it could be worthwhile to standardize these URLs to the following:

1. User goes to /gallery and lands on the current year.
2. User selects another year (say, 2018); the URL changes to /gallery/2018. Going back to the most recent year (2020, as of writing) would change the URL to /gallery/2020. This way, we could link out directly to particular years just as we link out to particular months.
3. User selects a month (August); the URL changes to /gallery/2020/august instead of /calendar/2020/august

We would need to set up routes for the years themselves in Rails as well as React Router, and it would probably be good practice to set up redirects for past gallery items so that old links aren't broken.

[atomtay]

The forthcoming visualization will have the URL /gallery/2021/january, as will future visualizations. A re-direct should be set up in order to convert the 2020 urls from /calendar to /gallery