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.
* Use email_address table to check user's email when login with email adress
* Update services/auth/signin.go
* Fix test
* Fix test
* Fix logging in with ldap username != loginname
* Fix if user does not exist yet
* Make more clear this is loginName
* Fix formatting
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>
