Change join slack button

[revelcw]

The submit email button looked weird to me so I changed it.

Before

After

[matthew-gerstman]

Hey Revel!

Thanks so much for opening a PR.

Unfortunately, I'd recommend against merging this one.

The bigger button is easier for touchscreen devices (regardless of screen size) so I'd like to keep it the way it is.

[donavon]

I think the old way (where the button is the same size as the input) looks like 2 input fields. That's always bugged me. In any case, on smaller screen sizes, it reverts back to the old way, so I'm not concerned. Also, the new way isn't a small button, just smaller.