Make assets task registered and configured by default

[erikjung]

Currently, the assets task from https://github.com/cloudfour/core-hbs-helpers is present as a dependency, but is not registered or configured in gulpfile.js or config.js respectively.

Since it's common to need images and other static assets copied into dist, this task should probably be enabled and ready to go.

Questions

• Which folder location makes the most sense for sourcing? We've used src/assets/toolkit/ before, but is that what we want as a default?

[megnotarte]

Small

[megnotarte]

@tylersticka doesn't have a strong opinion but is used to it under src. No one else seems to have a preference. @erikjung said he'll put it next to where the stylesheets for the toolkit live.