* 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>
