Use react-router to manage modals

[toolness]

As mentioned in #567 and alluded to in #373 (comment), we may want to consider giving modals their own routes, e.g. /mozilla-web-clubs/add for the "add a club" modal. This has a number of benefits:

• It allows react-router to do much of the work of showing/hiding modals.
• Links to modals can be shared between users. (For instance, we can share a direct link to the "add a club" modal, rather than sharing a link to the clubs page and telling users to click "add your club").
• It makes modals easier to develop and debug (devs can just go to the URL of the modal they want to change and reload it).
• Giving each modal its own URL could make it possible for non-JS and "safe mode" browsers to use modals.

[Pomax]

react-router has changed so much that we'll be looking at how to update the codebase afresh in 2016