Don't send overly large request from Frontend

[matthew-white]

Before sending a file, Frontend checks that the file doesn't exceed the nginx limit of 100 MB. However, Frontend only checks that for files, not other request bodies, so it's possible for other large requests to be sent, resulting in a 413 error. Instead, it'd be better for Frontend to check all request bodies before sending them. (Though I'm not quite sure how to implement that. Can we access the Content-Length header somehow?)

This issue is labeled as "blocked", because we don't want to make this change until #609 is resolved. For now, we're taking the approach described in #610: allowing the request to be sent, but showing an informative error message. However, once #609 is resolved, it'd be good to prevent the request from being sent in the first place.