Rename ENABLE_ALL_FEATURES flag to ENABLE_OPTIONAL_FEATURES for versions v2.3.0 and up

[bertdeblock]

The ENABLE_ALL_FEATURES flag was removed in v2.3.0 in favour of ENABLE_OPTIONAL_FEATURES:
emberjs/ember.js#12627

[locks]

Since it touches quite old versions, we will have to re-compile them. I think we should do this, but I'm marking as changes requested so someone doesn't accidentally merge this and we forget to precompile the old versions.
@mansona can you provide some guidance on how to do it?

[bertdeblock]

@locks Is re-compiling the older versions something we still want to do?

[bertdeblock]

Can we merge this and create a separate issue for re-compiling the older versions?

EDIT:

I missed this:

Please note that no attempt is made to update content for older versions of the Guides. Except for broken links, they are considered static and immutable, as it is too difficult to maintain content for every version ever released. Issues will only be fixed for future releases.

I can also just revert the changes for the older versions.

[locks]

sure