Repositories: by default disable all units except code and pulls on forks (#22541)

Most of the time forks are used for contributing code only, so not
having
issues, projects, release and packages is a better default for such
cases.
They can still be enabled in the settings.

A new option `DEFAULT_FORK_REPO_UNITS` is added to configure the default
units on forks.

Also add missing `repo.packages` unit to documentation.

code by: @brechtvl

## ⚠️ BREAKING ⚠️ 

When forking a repository, the fork will now have issues, projects,
releases, packages and wiki disabled. These can be enabled in the
repository settings afterwards. To change back to the previous default
behavior, configure `DEFAULT_FORK_REPO_UNITS` to be the same value as
`DEFAULT_REPO_UNITS`.

Co-authored-by: Brecht Van Lommel <brecht@blender.org>

modules/setting/repository.go

@@ -41,6 +41,7 @@ var (
 		EnablePushCreateOrg                     bool
 		DisabledRepoUnits                       []string
 		DefaultRepoUnits                        []string
+		DefaultForkRepoUnits                    []string
 		PrefixArchiveFiles                      bool
 		DisableMigrations                       bool
 		DisableStars                            bool `ini:"DISABLE_STARS"`
@@ -157,6 +158,7 @@ var (
 		EnablePushCreateOrg:                     false,
 		DisabledRepoUnits:                       []string{},
 		DefaultRepoUnits:                        []string{},
+		DefaultForkRepoUnits:                    []string{},
 		PrefixArchiveFiles:                      true,
 		DisableMigrations:                       false,
 		DisableStars:                            false,