feat: add support for React Router 6 stable

[kyletsang]

There were some breaking changes from beta to stable, so this PR fixes that.

Wasn't sure if we actually needed to render a Route inside the LinkContainer. It was causing errors in v6 stable. I think the hooks provided by React Router should be enough. I tested using the visual tests page and everything looks like it works.

[Coolfeather2]

Still not working for me inside my project, or in the visual test page
Inside the visual test page, no links are there. (At least the nav page is not working)

[kyletsang]

Still not working for me inside my project, or in the visual test page Inside the visual test page, no links are there. (At least the nav page is not working)

You downloaded the branch this PR is referencing and npm installed?

[mxschmitt]

LGTM, but next time split applying formatting changes with changing logic. Thanks!

[Coolfeather2]

1. git cloned
2. checked out react-router-v6-stable
3. ran npm install
4. ran npm run build
5. ran npm run visual-test
   

node -v
v14.15.4
npm -v
6.14.10

[Coolfeather2]

Additionally, when packaging this up, or using npm link it doesn't work

package.json is looking for ./lib/index.js, but ./lib/ReactRouterBootstrap.js is being built

[kyletsang]

Yeah the NavItem thing should be fine. There's supposed to be a NavLink underneath that gives it a little bit more styling

I'm going to merge this for now and address the build in another follow-up PR.