Added support for #at Hashtag

This was the first time i saw, that they're using "#at" instead of "#t",
so i've just added it.
Merge pull request #1324 from SoftCreatR/YTFix

[Youtube] Added support for #at Hashtag
Adds possibility to cancel installation

Closes #1078
Merge branch 'master' of github.com:WoltLab/WCF
Fixed issues when using old PCRE library version
Merge branch 'master' of github.com:WoltLab/WCF
Fixed issues when using old PCRE library version
Fixed temp folder during WCFSetup
Merge branch 'master' of github.com:WoltLab/WCF
Small style optimization
Fixed request method too long issue
Merge branch 'master' of github.com:WoltLab/WCF
Fixes sorting footer menu items
Adds BBCode permission validation for message previews
Updating minified JavaScript files
Added missing target=_blank
Merge branch 'master' of github.com:WoltLab/WCF
Fixes button label for native BBCodes
Merge branch 'master' of github.com:WoltLab/WCF
Fixed recaptcha module setting