Explicitly validate and cast dandiset ID during upload initialization

[sentry]

Sentry Issue: DANDI-API-R1

ValueError: invalid literal for int() with base 10: 'DANDI:000029'
  File "django/db/models/fields/__init__.py", line 2128, in get_prep_value
    return int(value)

ValueError: Field 'id' expected a number but got 'DANDI:000029'.
(18 additional frame(s) were not displayed)
...
  File "dandiapi/api/views/upload.py", line 135, in upload_initialize_view
    dandiset = get_object_or_404(

[jjnesbitt]

This is occurring because the dandiset_id referred to in this code snippet is not being cast to an integer first. We're passing django a string, and letting it do implicit coercion. We should 1.) validate that the string regex is in fact valid (not being done here, apparently), and 2.) explicitly coerce that value to an int.