Add a tool tip the the LMS URL assignment option that tells users that Google Drive links will not work

**What should this say?**
I'm open to just about any text, but I suggest the following:

> Links to PDFs in Google Drive will not work reliably with this option. If you're using a PDF stored in Drive, please use the "Select PDF from Google Drive" button.

This take would appear on hover or on click in a tool tip.

**Why do this?**
We've seen a few users try to circumvent syncing Drive to Hypothesis by copy/pasting links to PDFs in the URL option. The worst case scenario is when this works initially and then fails after they've already had a class annotate the document.

No files, tests, or entry points are named. Start by locating the LMS URL assignment option in the relevant application code, then confirm how existing tooltips are implemented. Done means the proposed warning appears on hover or click and distinguishes the Google Drive PDF selection flow.

Written by the indexing model from the issue text.