Add guard for frontend-based AuthenticationHelpers
Add guard for frontend-based AuthenticationHelpers

These Spree::AuthenticationHelpers methods should only be available for
apps that have `solidus_frontend` available, as they are simply proxies
for route helpers provided by that gem.

Closes #59.

solidusio/solidus_auth_devise,solidusio/solidus_auth_devise,AngelChaos26/mandrill,mayankdedhia/solidus_auth_devise,AngelChaos26/mandrill,mayankdedhia/solidus_auth_devise,AngelChaos26/mandrill,solidusio/solidus_auth_devise,mayankdedhia/solidus_auth_devise