Make disableLegacyExtensions overwriteable

Created by: philipp-spiess

We have to revert https://github.com/sourcegraph/sourcegraph/pull/40081 closer to the 4.0 release.

The issue with this flag is that it is defined as a boolean in a Go struct and thus initialized with false when not set in the JSON config. This caused an issue for us as we want to keep this false by default before the 4.0 release: only we should be able to turn extensions off until then for testing purposes.

We settled on making this flag static instead so that we can only change it via a code change. We can revert the PR I linked above closer to 4.0 to get a dynamic behavior with the proper default value.

/cc @muratsu @jjinnii @ryankscott