Klageautomat/Untätigkeitsklage: Cross-check entered dates against data of FOI request history

Follow-up of https://github.com/okfde/froide/issues/221

Feature request

The dates entered should be cross-checked against the dataset from the IFG request.
I.e. if I e.g. enter that I got an administrative decision (Bescheid) on 2020-12-12, but no such document is available in the FOI request, it should warn me to prevent accidental mistakes.

Also it could check whether there is a (PDF) attachment on that request and show a warning if there is no one.

What should happen

A warning (yellow box) like this:

Sie haben ein Datum angegeben, zu dem kein dazugehöriges Anfragedokument [mit angehängert PDF-Datei] gefunden wurde. Falls dieses noch nicht hochgeladen wurde oder ein falsches Datum angegeben wurde, so empfehlen wir, dies zunächst hochzuladen bzw. zu korrigieren, bevor sie weiter fortfahren.
[Trotzdem fortfahren]

I.e.

• warn about the problematic fact
• suggest to upload the document first or correct the date in the request if that is wrong
• still allow one to continue, if one e.g. did not want to upload it with the date or chose a somewhat different date (e.g. the date one has received the document vs it was written or so) or the agency really choose a non-PDF format or such arcane things

Reasoning/background

This helps to:

• spot easy mistakes/typos in your date
• prevent some easy mistakes, where you chose the wrong request date where you only received a notice and no formal PDF document

It's similar to the usual, manual complaint request process, where you also extract all dates and display it as a option list. There, one cannot even enter any non-existant date, but IMHO it is fine for the automatic one, as one may just have missed uploading it or does not want to upload it etc.

Start by tracing the automatic complaint request flow and compare it with the manual complaint request process described in the issue, including how request dates and PDF attachments are represented. The completed change should warn when an entered date has no matching request document or PDF attachment, while still allowing the user to continue.