Add loading state to submit button

[naomi-sharif]

When a user clicks the submit button on the Join page, there is no loading state, this means that the user is not aware if they have successfully submitted the form, thus resulting in multiple submissions.
Upon form submit, the button and the form fields should be disabled.
For the button disabled styling, I would suggest reducing the opacity and also changing the text to say 'Submitting'.
Once the request has been submitted, the user should then be shown the Success page, see issue #11
All styling to be following BEM

[ShawnMorreau]

I have opened a pull request for this issue, does not solve issue #11 though
@naomi-sharif