Add check to verify token was received.
Update headers sent to content API.
Add availability check for videos to resolve last part of #472 where only SD version is available.
Cover edge cases that shouldn't ever happen but probably will sometime.
Also included Imgur changes to cover similar situations of malformed/redirected links.
API seems to return incorrect signature value when sending header. Other fixes seems to have worked temporarily but have stopped working so they're removed.
```
File "/home/xk/github/o/bulk-downloader-for-reddit/bdfr/connector.py", line 413, in check_subreddit_status
assert subreddit.id
File "/home/xk/.local/share/virtualenvs/bulk-downloader-for-reddit-dCAFmVJi/lib/python3.10/site-packages/praw/models/reddit/base.py", line 34, in __getattr__
self._fetch()
File "/home/xk/.local/share/virtualenvs/bulk-downloader-for-reddit-dCAFmVJi/lib/python3.10/site-packages/praw/models/reddit/subreddit.py", line 584, in _fetch
data = self._fetch_data()
File "/home/xk/.local/share/virtualenvs/bulk-downloader-for-reddit-dCAFmVJi/lib/python3.10/site-packages/praw/models/reddit/subreddit.py", line 581, in _fetch_data
return self._reddit.request("GET", path, params)
File "/home/xk/.local/share/virtualenvs/bulk-downloader-for-reddit-dCAFmVJi/lib/python3.10/site-packages/praw/reddit.py", line 885, in request
return self._core.request(
File "/home/xk/.local/share/virtualenvs/bulk-downloader-for-reddit-dCAFmVJi/lib/python3.10/site-packages/prawcore/sessions.py", line 330, in request
return self._request_with_retries(
File "/home/xk/.local/share/virtualenvs/bulk-downloader-for-reddit-dCAFmVJi/lib/python3.10/site-packages/prawcore/sessions.py", line 266, in _request_with_retries
raise self.STATUS_EXCEPTIONS[response.status_code](response)
prawcore.exceptions.Redirect: Redirect to /subreddits/search
```
