feat(timers): Redesign timers and remove dependency on day.js

[Hanziness]

This PR moves the responsibility of time formatting into the timer modules, allowing the removal of the dependency on day.js. This is great for multiple reasons:

1. Timer formatting is implemented locally in the timer display modules (they can be tweaked, unlike with day.js)
2. The approximate timer can be translated along with the other strings
3. There's no need to sync the day.js language option with vue-i18n's locale.
4. Bundle sizes and the number of dependencies are lower

Also, all three timers received an updated design. Without the use of tabular numbers (and the Source Sans Pro font), the traditional and percentage timers can still display equal-width numbers. This makes the design look more consistent and further reduces the bandwidth required by the app.

Closes #163
Closes #170