[Site feature request] Support for Kemono DMs

[Fukitsu]

Some artists send links to download the rewards in DMs and kemono has support for those but gallery-dl doesn't seem to be able to scrape them (as far as I know)

My config for kemono metadata is:
"postprocessors": [ { "name": "metadata", "event": "post", "filename": "info.json" } ]

It'd be good if we could save those to the metadata file.

Example https://kemono.party/patreon/user/5490657/dms

[kattjevfel]

So I see that an option for DMs were added with c67756e, but this issue was never closed. I tried enabling it just now and it doesn't appear to be working (anymore?):

katt@main ~ » gallery-dl --ignore-config -o extractor.kemonoparty.dms=true --verbose https://kemono.party/patreon/user/3161935
[gallery-dl][debug] Version 1.20.3-dev
[gallery-dl][debug] Python 3.10.2 - Linux-5.17.0-rc1-1-mainline-x86_64-with-glibc2.33
[gallery-dl][debug] requests 2.26.0 - urllib3 1.26.8
[gallery-dl][debug] Starting DownloadJob for 'https://kemono.party/patreon/user/3161935'
[kemonoparty][debug] Using KemonopartyUserExtractor for 'https://kemono.party/patreon/user/3161935'
[urllib3.connectionpool][debug] Starting new HTTPS connection (1): kemono.party:443
[urllib3.connectionpool][debug] https://kemono.party:443 "GET /api/patreon/user/3161935?o=0 HTTP/1.1" 200 8386
[urllib3.connectionpool][debug] https://kemono.party:443 "GET /patreon/user/3161935/dms HTTP/1.1" 200 2934
[kemonoparty][error] An unexpected error occurred: AttributeError - 'NoneType' object has no attribute 'strip'. Please run gallery-dl again with the --verbose flag, copy its output and report this issue on https://github.com/mikf/gallery-dl/issues .
[kemonoparty][debug] 
Traceback (most recent call last):
  File "/usr/lib/python3.10/site-packages/gallery_dl/job.py", line 79, in run
    for msg in extractor:
  File "/usr/lib/python3.10/site-packages/gallery_dl/extractor/kemonoparty.py", line 77, in items
    dms = self._extract_dms(post)
  File "/usr/lib/python3.10/site-packages/gallery_dl/extractor/kemonoparty.py", line 184, in _extract_dms
    )[0].strip()),
AttributeError: 'NoneType' object has no attribute 'strip'