Make the submission form a proper <form>

[winstliu]

As promised in the previous PR, made the answer textbox + button combo into an actual <form>. In addition to the standard accessibility benefits of having screenreaders go "oh, this is a form", we can simplify a lot of the JS handling. Instead of having to add event handlers for both the textbox & the button, now we can listen to the form's 'submit' event and override that.
Also tweaked the styles a bit. If the before/after look very similar, that's intentional :). Pretty much the only visible differences are:

• "Answer:" is now centered
• There's now some margin between the submission form & the previous answers dropdown

Before:

Before (freeform):

Before (answer submission):

After:

After (freeform):

After (answer submission):

[winstliu]

...pretty sure all this ASP stuff is dead, considering the page manually hits an API endpoint, rather than sending a POST back?

[tabascq]

Thanks - I would like @morganbr or @asyasky to look at this one because I don't completely understand the freeform part.