allow use without solidus_backend

[ccarruitero]

Summary

Currently solidus_subscriptions has references to solidus_backend, even when solidus_backend isn't a dependency on gemspec.

It will be easier just add solidus_backend as dependency in gemspec, but I think the plan with solidus_admin is to eventually replace solidus_backend. So, this PR allows use solidus_subscriptions extension without solidus_backend.

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[fthobe]

@jarednorman whats the general opinion on this?

[jarednorman]

PR needs fixing, but we want this.

[fthobe]

@jarednorman ok, we have added it to the list and will tackle this in a bugfix run we surely need in march / april.

[fthobe]

@jarednorman do you have information to add here?