Use actual ISO8601 timestamps for masto API (#425)

Use actual ISO8601 timestamps for masto API (#425)

Some users post posts with spoofed timestamp, and some clients will have issues with certain dates. Tusky for example crashes if the date is any sooner than 1 BCE (“year zero” in the representation).

I limited the range of what is considered a valid date to be somewhere between the years 1583 and 9999 (inclusive).

The numbers have been chosen because:

- ISO 8601 only allows years before 1583 with “mutual agreement”
- Years after 9999 could cause issues with certain clients as well

Co-authored-by: Charlotte 🦝 Delenk <lotte@chir.rs>
Reviewed-on: https://akkoma.dev/AkkomaGang/akkoma/pulls/425
Co-authored-by: darkkirb <lotte@chir.rs>
Co-committed-by: darkkirb <lotte@chir.rs>