Replace #25580
Fix #19453
The problem was: when users set "GITEA__XXX__YYY" , the "install page"
doesn't respect it.
So, to make the result consistent and avoid surprising end users, now
the "install page" also writes the environment variables to the config
file.
And, to make things clear, there are enough messages on the UI to tell
users what will happen.
There are some necessary/related changes to `environment-to-ini.go`:
* The "--clear" flag is removed and it was incorrectly written there.
The "clear" operation should be done if INSTALL_LOCK=true
* The "--prefix" flag is removed because it's never used, never
documented and it only causes inconsistent behavior.
The INI package has many bugs and quirks, and in fact it is
unmaintained.
This PR is the first step for the INI package refactoring:
* Use Gitea's "config_provider" to provide INI access
* Deprecate the INI package by golangci.yml rule
Replace #19857
Close #19856
Close #10311
Close #10123
Major changes:
1. Move a lot of code from `environment-to-ini.go` to `config_env.go` to
make them testable.
2. Add `__FILE` support
3. Update documents
4. Add tests
