Add lookback.io to the internal beta

This is sort of minor, please bear with me. 😁

I noticed how we end up having several `ifdefs` like the following one:

`#if defined(DEBUG) || defined(INTERNAL_BETA)`

There's a pretty minor disadvantage in having this: if we decide to, for example, disable the feature for internal builds, we need to go through the code and find the spots where these `ifdefs` are.  This could take a while, as there may be other features using the same `ifdef`.

A nice alternative to this would be to have a specific flag, like `LOOKBACK_ENABLED`, that when defined enables the feature.  This makes configuration easier, as we'd just need to define the flag in the project settings for the targets we want this feature to be enabled in.  This also means that in order to change what targets this feature is in, we can just do so from the project settings.