Check for spree_frontend gem via Gem::Specification

[nciemniak]

Currently, the spree_auth_devise code checks for the spree_frontend gem by checking if Spree::Frontend::Engine is present. This makes it so that if spree_auth_devise is listed in the gemspec before spree_frontend within a user's gemfile, the spree auth code will not find spree_frontend.

Checking for spree_frontend via Gem::Specification allows the code to know whether the spree_frontend gem is present whether or not it has been initialized already or not.

This fixes the broken /account page in the spree legacy frontend.

[nciemniak]

Testing completed:

• check that spree_starter successfully runs and does not throw error on /account page when spree_auth_devise gem is listed before spree_frontend in the gemfile
• step through code in spree_auth_devise around frontend_available? method to ensure the code is able to successfully load all necessary classes during route and engine initialization

Should be ready to merge @rafalcymerys

Will be making a separate PR to also update the methods checking for backend and API, but would prefer to apply those changes after the 4.5 release to allow more time for the changes to be tested.