Make the "add to mixin config" intention respect the target class' side/environment

[Earthcomputer]

Currently the mixin is always added to the mixins array in mixin configs, however if the target class is client-side-only, it should be easy to infer that it should go in the client array instead.

[RedNesto]

I implemented this based on the SideOnly information we have access to (893b815558d3553b79d58a9615ff932b7d2d8caa). Dunno if this is enough to close this issue.