Add enableLegacyExtensions

Created by: vdavid

Closes https://github.com/sourcegraph/sourcegraph/issues/39429

Closely followed Taras's https://github.com/sourcegraph/sourcegraph/pull/39136 Also added a new React hook to make it easy to access the setting.

As for @philipp-spiess's comment “I think this should be an instance wide setting not a user specific one.” → good point, and in my best understanding, both Taras's and this new flag are instance-wide only, they can only be set in Global settings and not anywhere else.

Test plan

Nothing depends on this, so not much. See the screenshot that it can be set. Autocomplete also lists it.