Ensure our links to blank targets have the proper "rel" attribute set

[toolness]

While working on https://github.com/JustFixNYC/who-owns-what/pull/95 I ran into the jsx-no-target-blank rule and realized that the tenants app probably doesn't adhere to it, and it should.

While we're at it, it might be useful to actually run eslint with this kind of rule enabled.

[toolness]

I recently saw that Chrome and other browsers are finally applying a default rel="noopener" to anything that opens a link with target="_blank", at least, so that's good: https://www.chromestatus.com/feature/6140064063029248