MU blueprints: enable EMBER_MODULE_UNIFICATION feature flag

[ppcano]

Thanks to @patricklx for finding and providing the fix of this issue at #8424 (comment)

The MU blueprints were not enabling the EMBER_MODULE_UNIFICATION Ember Feature Flag. This PR enables it (I will also add it to the octane blueprints).

Relates #8431 and #8424

---

I have reviewed the Ember code and it looks that there is an issue regarding how Ember Feature Flags works: the Feature Flags doc and guide looks to indicate that the feature key should be in dash and lowercase format, but I could not find in the Ember code that a key transformation from lowercase to uppercase format.

Should it be enabled in lowercase and fix the issue or should the documentation be updated?

[rwjblue]

Should it be enabled in lowercase and fix the issue or should the documentation be updated?

The documentation should be updated. When it was originally authored we did use lower case dasherized keys, now we use SCREAMING_SNAKE_CASE. The main reason for the change was that with SCREAMING_SNAKE_CASE we can use them directly as identifiers in javascript/typescript code whereas foo-bar-baz cannot be.