Add support for @ prefix on MentionFilter base_url

[jeremysmithco]

I had a situation where I wanted to use the @ prefix on mention links with the MentionFilter (the way tilde is already supported). Example: /@username.

Currently, if you set the base_url like this: base_url: "/@", then the link would be: /@/username and I don't want the slash between @ and username.

I've updated MentionFilter#link_to_mentioned_user and TeamMentionFilter#link_to_mentioned_team to support this, and added accompanying tests.

Additionally, I noticed that the regex %r{[/~]\z} will match strings like ~~~ and //// and I'm wondering if that's intended? If not, I could change those checks to something like url.ends_with?("/@").

Thanks for this gem! 😊

[gjtorikian]

Hey thanks for this! I simplified the regexp, and can put this out in a patch release after I review your other PR.

[jeremysmithco]

Hey thanks, much appreciated! 😊