Improve comment links

[matthijskooijman]

This adresses some problems with the comment links feature merged in #356, as discussed in https://github.com/PlanBCode/hypha/pull/356#issuecomment-743437016 onwards.

This PR contains the commits previously pushed by @RoukePouw, but I:

• Split his first commit into two
• Added a fix for php-dom-wrapper's create method eating up text not wrapped in a span or so. I've suggested this same fix upstream, and I'd like to hear their response before merging this here (ideally, we could just backport a fix with an upstream commit reference as we've done before). See https://github.com/scotteh/php-dom-wrapper/issues/15#issuecomment-750145412
• Limited the regex to require either http://, https:// or a www prefix, to prevent matching words with a dot halfway.
• Added _ to the allowed characters in urls
• Removed the non-greedy *? matching and removed the + from the trailing non-dot, since that would prevent a domain with three components (e.g. https://www.dewar.nl) from being matched.
• Removed one reference to the now-removed $suffix variable.
• Rebased on top of master.

@RoukePouw, how does this look? If this looks ok to you and the php-dom-wrapper maintainer applies this fix upstream, I think this addresses everything?

[RoukePouw]

Looks great! :)

[matthijskooijman]

I've rebased this on master, and squashed the fixups. I want to go ahead and merge this even though phpdomwrapper's maintainer has not responded yet, we can always update our fix when they do.

@RoukePouw, I also noticed that in one of your commits, you removed the u modifier on the regex, which enables UTF-8 processing. Was that intentional? It looks like having that modifier is correct, so I modified your commits to leave it now.