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/translation/i18n/i18n_test.go

@@ -0,0 +1,56 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package i18n
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_Tr(t *testing.T) {
+	testData1 := []byte(`
+.dot.name = Dot Name
+fmt = %[1]s %[2]s
+
+[section]
+sub = Sub String
+mixed = test value; <span style="color: red\; background: none;">more text</span>
+`)
+
+	testData2 := []byte(`
+fmt = %[2]s %[1]s
+
+[section]
+sub = Changed Sub String
+`)
+
+	ls := NewLocaleStore()
+	assert.NoError(t, ls.AddLocaleByIni("lang1", "Lang1", testData1))
+	assert.NoError(t, ls.AddLocaleByIni("lang2", "Lang2", testData2))
+	ls.SetDefaultLang("lang1")
+
+	result := ls.Tr("lang1", "fmt", "a", "b")
+	assert.Equal(t, "a b", result)
+
+	result = ls.Tr("lang2", "fmt", "a", "b")
+	assert.Equal(t, "b a", result)
+
+	result = ls.Tr("lang1", "section.sub")
+	assert.Equal(t, "Sub String", result)
+
+	result = ls.Tr("lang2", "section.sub")
+	assert.Equal(t, "Changed Sub String", result)
+
+	result = ls.Tr("", ".dot.name")
+	assert.Equal(t, "Dot Name", result)
+
+	result = ls.Tr("lang2", "section.mixed")
+	assert.Equal(t, `test value; <span style="color: red; background: none;">more text</span>`, result)
+
+	langs, descs := ls.ListLangNameDesc()
+	assert.Equal(t, []string{"lang1", "lang2"}, langs)
+	assert.Equal(t, []string{"Lang1", "Lang2"}, descs)
+}