Youtube embed not rendering in Safari

## Details
- OS (Operating System) version: macOS 12.5.1 (Monterey)
- Browser and browser version: Safari 16.0
- Django version: 4.1.2
- Martor version & theme: 1.6.14 with Bootstrap theme

### Steps to reproduce
1) **In Safari**: In the Simple Form demo, just pasting a Youtube url in the markdown editor, renders an empty iframe.

2) As you can see, the the src parameter is set without the protocol string (https).

3) This works fine in Chrome (106.0.5249.103) and Firefox (v 105.0.3)

I know this is a bug report, but in case it helps, I made the following changes to the mdx_video.py file

1) In the extendMarkdown function, I changed the regular expression to capture the protocol along with the youtubeid parameter. So the code was change from
`r"([^(]|^)https?://www\.youtube\.com/watch\?\S*v=(?P\S[^&/]+)"`
TO
`r"([^(]|^)(?P(https?))://www\.youtube\.com/watch\?\S*v=(?P\S[^&/]+)"`

2) In the Youtube class, I changed the code from
```
class Youtube(markdown.inlinepatterns.Pattern):
def handleMatch(self, m):
if self.ext.config["youtube_nocookie"][0]:
url = "//www.youtube-nocookie.com/embed/%s" % m.group("youtubeid")
else:
url = "//www.youtube.com/embed/%s" % m.group("youtubeid")
width = self.ext.config["youtube_width"][0]
height = self.ext.config["youtube_height"][0]
return render_iframe(url, width, height)
```
TO the version below that just uses the protocol parameter
```
class Youtube(markdown.inlinepatterns.Pattern):
def handleMatch(self, m):
if self.ext.config["youtube_nocookie"][0]:
url = "{0}://www.youtube-nocookie.com/embed/{1}".format(m.group("protocol"), m.group("youtubeid"))
else:
url = "{0}://www.youtube.com/embed/{1}".format(m.group("protocol"), m.group("youtubeid"))
```

Finally, thank you so much for your work/plugin. It has been incredibly helpful. If you take pull requests, I'd be happy to make the changes for your review.