xkcd: account for Bing search returning mobile URLs

[dgw]

Description

Yeah, .xkcd search hasn't been working very well because Bing decided to start returning m.xkcd.com URLs much of the time.

Interestingly, adding -site:m.xkcd.com made Bing return NO results, not the normal (non-mobile) link. That's why this patch modifies the regex to validate the result instead.

I'm not sure what's up with the optional protocol prefix, but didn't want to touch it in a patch intended for stable branch.

Effect

# Before (v7.1.7)
11:20:16 <dgw> .xkcd team chat
11:20:16 <Sopel> dgw: Could not find any comics for that query.

# After (patch head, b706c0e841a35d095c549825ec4a08bc2bf13af5)
11:27:01 <~dgw> ,xkcd team chat
11:27:03 <SopelTest> [xkcd] Team Chat | Alt-text: 2078: He announces that he's finally making the jump from
                     screen+irssi to tmux+weechat. | https://xkcd.com/1782

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

Notes

Back in November I tried to fix this on the search plugin side with a few never-submitted tweaks that didn't actually help reliably. This solution is so stupid-simple… 😅

Along the way I considered trying to make the search more robust by pulling in results from both sopel.modules.search.bing_search() and sopel.modules.search.duck_search(), but that's probably overkill.

[Exirel]

I mean... that's a hotfix, not a rewrite, so yeah, all good.