Handle Invalid Date responses

[tim-macphail]

With the introduction of the /premium-files endpoint, the openai api sometimes doesn't give a valid date in the response.
E.g. the response.assessments = {name: "Final exam", weight: 30, date: null}

This needs to be handled properly. Right now, it will load into the frontend and sit at the bottom of the assessments list without throwing any errors. A date can be manually added to it by editing it. However, if one doesn't add a date before trying to export, uncaught errors will be thrown in icsGen.ts

This can be recreated by uploading CPSC331.pdf sometimes.

We need to decide how to handle this. Options
1. In the server, drop assessments with invalid dates.

• pros: requires no change to frontend, easy to implement
• cons: the assessment often does exist but just doesn't have a date stated in the outline, so it might be useful to keep that info

2. In client, skip over assessments with no date when generating iCal. Could even have a toast/alert notifying user that this is happening

• pros: fixes errors
• cons: assesment is dropped

3. In client, don't allow export until date field has a value

• pros: no assessment slips through the cracks
• cons: harder to implement. Definitely would require Delete assessment #162 to be done

Edit: We'll go with 3. Blocked by #177