Remove two unmaintained vendor packages `i18n` and `paginater`. Changes:
* Rewrite `i18n` package with a more clear fallback mechanism. Fix an unstable `Tr` behavior, add more tests.
* Refactor the legacy `Paginater` to `Paginator`, test cases are kept unchanged.
Trivial enhancement (no breaking for end users):
* Use the first locale in LANGS setting option as the default, add a log to prevent from surprising users.
Fix#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.
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>
