Fix article edit form.

[jglamine]

Fixes #585

Right now, it's impossible to edit articles. A JavaScript error is thrown when the page loads, preventing the form's onsubmit handler from loading.

As a result, the 'submit' button posts to /submit (which does not exist) instead of making an xhr post to /api/submit.

[jglamine]

I've tested the following:

• Edit post on main
• Edit post on discussion
• Submit link on main
• Submit link on discussion
• Submit article on main
• Submit article on discussion
• Start on the link tab, then switch to the article tab and submit
• Start on the article tab, then switch to the link tab and submit
• Promote an article
• Promote a link

Everything looks good to go.

[vipulnaik]

Thanks for fixing this, @jglamine -- I really appreciate it.