[6.x] Use CommonMark For Mailable

[taylorotwell]

This switches Markdown Mailable parsing to CommonMark instead of Parsedown. The main reason for this is better granularity of security related features in CommonMark, which allows us to prevent unsafe links in user input (a user input with the content: "a") without needing to prevent all HTML tags in general which would break our current mailable system.

I think it would be good to do this basically security related change on 6.x because it is an LTS.

[shengslogar]

It's worth noting that (new Parsedown())->text('#Header') (note the missing space after the octothorpe) will "correctly" translate to <h1>Header</h1>, while (new League\CommonMark\CommonMarkConverter())->convertToHtml('#Header') translates to <p>#Header</p>.

The correct syntax, of course, is # Header (with the added space), and will translate properly with both libraries.

There are also other small nuanced differences. Parsedown translates - to <p>-</p>, while CommonMark translates - to <ul>\n<li></li>\n</ul>\n.

[driesvints]

@shengslogar actually, that's not correct. Any hashtag character with no spaces after it is interpreted as a regular hashtag. See the spec here: https://spec.commonmark.org/0.29/#example-34

Otherwise you could never start a sentence with a hashtag. This was something that Parsedown incorrectly rendered.

Edit: just realized you probably meant the same by putting "correctly" between quotes.

[shengslogar]

@driesvints Right on. I slipped up in a project, which broke when I updated Laravel, but you're absolutely correct regarding the spec. Figured my comment might give pointers to someone with a similar issue.