Update Missing Syllabi Download Button

Issue Description: When a user goes to Administration → Course Management and uses the Missing Syllabi section, the button is labeled Submit, even though its actual action is to download a spreadsheet.

Update the button label from Submit to Download so the action is clear. The button should also be disabled when no options are selected, since clicking it without a selection currently downloads an empty spreadsheet.

Expected Behavior:
The button should be labeled Download and should only become active when at least one option is selected. If nothing is selected, the button should remain disabled and no empty spreadsheet should be downloaded.

UI: