Add full timestamps when hovering over relative times (e.g. "3 months ago")

[jake-low]

This is a small PR which adds title text (which desktop browsers show on hover) to relative timestamps in the UI, showing the absolute time. This is helpful when the UI shows things like "2 years ago" but you want to know the specific day that the change was made.

Example when hovering over a changeset timestamp:

I added a new RelativeTime component to contain this bit of logic, and changed everywhere that displays relative times to use this component.

This is my first PR to this project. I ran yarn precommit to lint and format the changes, but let me know if there are style changes I should make.

P.S. I attempted to follow the instructions in the README about local development, but they seem slightly out of date. I needed to do two things before I was able to test my changes locally:

1. Run the project with HTTPS=true yarn start (without this env variable the server listened for plain HTTP only)
2. Use the console on osmcha.org to retrieve the token value from localStorage, and setting that value on localhost too. I wasn't able to use the Sign In button on localhost; it seemed to sign me in on osmcha.org but not on the local development instance.