Backport #15452Fix#14793.
The previous implementation used the first return value of matcher.Match, which is the chosen language tag but may contain extensions such as de-DE-u-rg-chzzzz.
As mentioned in the documentation of language package, matcher.Match also returns the index of the supported tags, so I think it is better to use it rather than manipulate the returned language tag.
Co-authored-by: Naohisa Murakami <tiqwab.ch90@gmail.com>
Add SameSite setting for cookies and rationalise the cookie setting code. Switches SameSite to Lax by default.
There is a possible future extension of differentiating which cookies could be set at Strict by default but that is for a future PR.
Fix#5583
Signed-off-by: Andrew Thornton <art27@cantab.net>
