Support arbitrary asset upload/deletion for Library Components

[ormsbee]

#1398 specifies the ability to add images to an HTML or Problem component via the TinyMCE editor.

But for both debugging and for power user purposes, we're going to want to be able to do direct file upload via the Details tab in the sidebar. This will also allow ProblemBlocks to use static assets when using the advanced editor (which currently has no menu for asset upload). It will also allow for other kinds of assets to be uploaded and linked, like PDFs.

This can go directly under the part of the UI where we already display assets:

Quoting @bradenmacdonald:

adding a basic "drop file here to upload" to the power user assets section is probably very little work. You can just add a link that displays a ModalDropzone and then call this existing REST API to save the upload.

[ormsbee]

Ideally, we could also get delete buttons of some sort for the existing assets.

[bradenmacdonald]

@jmakowski1123 @sdaitzman @marcotuts This is ready for acceptance testing on the sandbox. I included a delete button.

(n.b. PR hasn't merged yet)

[ChrisChV]

@jmakowski1123 @sdaitzman @marcotuts Friendly reminder that this is ready for AC testing on the sandbox