* Refactor install page (db type)
* set correct default DB HOST for different DB TYPE
* remove legacy TiDB from documents
* unify the usage of DB TYPE, in code we only use "mysql". "MySQL" is only shown to users for friendly name.
* Gitea can use TiDB via MySQL protocol
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
* 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).
