urlpreview: Don't fetch previews for URLs in code blocks

[peterrus]

When someone embeds an URL in a code block (backtick or triple backtick) we probably shouldn't try to fetch a preview because that URL is there for different purposes than a regular URL that is posted to a channel.

I tried to do this with a regex, but my regex wizardry doesn't cut it unfortunately.

[coffeebank]

Thinking a possible way is to run two Regex commands:

1. The first one remains the same, parsing all instances of https://
2. The second one parses for all instances of (`|```) ... https:// ... (`|```) and returning the urls

Then, for every Regex 1 result, if result is in Regex 2, we ignore it

Downsides: Very inefficient, fails if users use ` as a regular quote, relies on parsing ` specifically (and not <pre> <code> etc.)

[peterrus]

Yeah this one seems quite hard. Maybe someone comes along with a good idea!