Config system overhaul

[silverwind]

Feature Description

Gitea's config system has various issues:

1. Environment variables are not recognized, unless environment-to-ini is loaded, which currently only runs for docker deployments.
2. There are two places to define a default value and it causes bugs, as seen in this PR.
3. There is ambiguity between casing, e.g. Enabled in the config struct, vs ENABLED in the .Key definition.

I would propose treating environment variables as first-class, so the following steps could be taken:

1. Move environment-to-ini into the main code base so environment variables are recognized at all times.
2. Use https://github.com/kelseyhightower/envconfig to parse the environment to config structs. The structs should have original case and format of the variables, so a config variable remains greppable, so it should be config.APP_NAME instead of config.AppName.
3. Have a separate mechanism that parses the ini and map values to the already initialized config struct, but not overwrite existing values because environment variables must have precedence over ini values.

[lunny]

I will against to support these hundreds of envs in Gitea directly. It will become unmaintainable thing.

[silverwind]

I think at minimum, we want to merge environment-to-ini into the codebase. It's just not acceptable to require a config file in this day and age.

It should be possible to start gitea in systemd with a bunch of variables in the gitea.service file only. See the gitpod config which had to specifically work around this silly requirement for a app.ini.

[wxiaoguang]

There are two places to define a default value and it causes bugs, as seen in this PR.

That's also related to the ini package's bug / quirk behavior. Sooner or later, the ini package should be written first before other config related refactoring.