Breaking change for integrators: Split the project up per fork

spree_bootstrap_frontend currently contains overrides for different parts of spree and spree_auth_devise. This causes some problems for pushing these changes back to the upstream projects. This is especially true for the spree_auth_devise changes as spree_auth_devise is a separate repo and requires a whole other PR/fork.

This is the start of the work to split the overrides for each project.

As part of this PR I will be splitting out the spree_auth_devise overrides, and possibly spree_core.

For more details see spree/spree@5044.