Remove legacy unmaintained packages, refactor to support change default locale (#19308)

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.

modules/markup/markdown/toc.go

@@ -8,7 +8,8 @@ import (
 	"fmt"
 	"net/url"
 
-	"github.com/unknwon/i18n"
+	"code.gitea.io/gitea/modules/translation/i18n"
+
 	"github.com/yuin/goldmark/ast"
 )