Improve install code to avoid low-level mistakes. (#17779)

* Improve install code to avoid low-level mistakes.

If a user tries to do a re-install in a Gitea database, they gets a warning and double check.
When Gitea runs, it never create empty app.ini automatically.

Also some small (related) refactoring:

* Refactor db.InitEngine related logic make it more clean (especially for the install code)
* Move some i18n strings out from setting.go to make the setting.go can be easily maintained.
* Show errors in CLI code if an incorrect app.ini is used.
* APP_DATA_PATH is created when installing, and checked when starting (no empty directory is created any more).

modules/setting/i18n.go

@@ -0,0 +1,51 @@
+// Copyright 2021 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 setting
+
+// defaultI18nLangNames must be a slice, we need the order
+var defaultI18nLangNames = []string{
+	"en-US", "English",
+	"zh-CN", "简体中文",
+	"zh-HK", "繁體中文（香港）",
+	"zh-TW", "繁體中文（台灣）",
+	"de-DE", "Deutsch",
+	"fr-FR", "français",
+	"nl-NL", "Nederlands",
+	"lv-LV", "latviešu",
+	"ru-RU", "русский",
+	"uk-UA", "Українська",
+	"ja-JP", "日本語",
+	"es-ES", "español",
+	"pt-BR", "português do Brasil",
+	"pt-PT", "Português de Portugal",
+	"pl-PL", "polski",
+	"bg-BG", "български",
+	"it-IT", "italiano",
+	"fi-FI", "suomi",
+	"tr-TR", "Türkçe",
+	"cs-CZ", "čeština",
+	"sr-SP", "српски",
+	"sv-SE", "svenska",
+	"ko-KR", "한국어",
+	"el-GR", "ελληνικά",
+	"fa-IR", "فارسی",
+	"hu-HU", "magyar nyelv",
+	"id-ID", "bahasa Indonesia",
+	"ml-IN", "മലയാളം",
+}
+
+func defaultI18nLangs() (res []string) {
+	for i := 0; i < len(defaultI18nLangNames); i += 2 {
+		res = append(res, defaultI18nLangNames[i])
+	}
+	return
+}
+
+func defaultI18nNames() (res []string) {
+	for i := 0; i < len(defaultI18nLangNames); i += 2 {
+		res = append(res, defaultI18nLangNames[i+1])
+	}
+	return
+}