Show add club modal on page load if ?modal=add is in url on clubs page.

[toolness]

This fixes #566.

Note that it doesn't remove ?modal=add when the modal is dismissed. This is partly because it's a hassle to implement and test, but also partly because I'd like to implement a more robust solution for giving every modal its own URL in v2, which I allude to a bit in #373 (comment) (this will make it a lot easier to allow older/non-JS browsers to use the modals too, among other things). As such, this PR is really just a quick fix to unblock mozilla/id.webmaker.org#51.

[ScottDowne]

R+ ✅ ✅ ✅

We could use this for the "add club" link at the bottom of the club toolkit page.

I gave it a quick look but it looks like it needs to use the Link component and that's not a simple linkTo += "?modal=add"so I say punt it to a new issue.

[toolness]

Thanks Scott!

I gave it a quick look but it looks like it needs to use the Link component and that's not a simple linkTo += "?modal=add"so I say punt it to a new issue.

Yeah, in v2 I'd actually like to change all modals have their own routes, e.g. /clubs/add for the clubs modal; I explain this a bit more in #373 (comment) but the gist is that it'll make react-router do more of the heavy lifting for us, it'll make the code more "react-esque" and easy to reason about, and it'll make it possible to make the modals accessible for folks without JS.

For now though I think it's best to keep ?modal=add's usage limited to a "hack" that allows our login flow to work--for instance, just turning the "add your club" into a <Link> won't actually work with this PR because that JS is only triggered in componentDidMount--meaning that clicking on the Link won't actually trigger the modal, because the component is already mounted! And I don't think that'd be the last of our problems, either... Giving every modal its own route in v2 will totally fix this, though.