Make "install page" respect environment config (#25648)

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.


![image](12778ee4-3fb5-4664-a73a-41ebbd77cd5b)

options/locale/locale_en-US.ini

@@ -296,6 +296,8 @@ invalid_password_algorithm = Invalid password hash algorithm
 password_algorithm_helper = Set the password hashing algorithm. Algorithms have differing requirements and strength. The argon2 algorithm is rather secure but uses a lot of memory and may be inappropriate for small systems.
 enable_update_checker = Enable Update Checker
 enable_update_checker_helper = Checks for new version releases periodically by connecting to gitea.io.
+env_config_keys = Environment Configuration
+env_config_keys_prompt = The following environment variables will also be applied to your configuration file:
 
 [home]
 uname_holder = Username or Email Address