Saving application multiple times breaks S3 upload

[epicfaace]

1. Upload a resume in your application.
2. Click "Save for later".
3. Click "Save for later".
4. Refresh the page.
5. The actual URL is saved in the forms.application_info.resume field instead of the S3 key.

Issue was caused by adding mongoose hooks on 12/5.

[epicfaace]

Solution: use a regex to extract the application ID from the S3 URL before saving it. It's a hacky solution for now.

Long-term solutions:

• In the pre-save hook, use something else to extract the application ID; maybe keep track of what the existing value for the file is before updating it.
• Don't use injectDynamicApplicationContent at all; the client receives the S3 object key, then opens an iframe to an endpoint such as api.treehacks.com/files/askldjklfj-forms.application.info which serves as a proxy to S3.