Variants were not being properly handled in the `flag-resolver`: The
fact that the default value of the variant is not falsy made it so we
never asked the external flag resolver for the value.
This also moves the logic from `Variant | undefined` to `Variant` where
we use the `getDefaultVariant()` helper method to return us a [default
variant](55274e4953/src/variant.ts (L37-L42)).
Adding stricter types to `FlagResolver` can possibly help improve our DX
- Help us prevent errors like typos, guide us to correctly add a flag
when needed, and warn us of stray checks whenever we do a clean up at a
later stage.
* fix: remove unused exp flag
* fix: remove unused flag
* fix: add support for external flag resolver
* fix: rename flagsresolver to flagresolver
* fix: disable external flag resolver
* fix: refactor a bit
* fix: stop using unleash in server-dev
* fix: remove userGroups flag
* fix: revert bumping frontend