Rename solidus_backend to solidus_admin

[benjaminwil]

The name solidus_backend creates communication problems between developers and non-developers. Developers have a very specific idea of what "backend" means when talking about web development, and it has almost nothing to do with solidus_backend. Let's call it solidus_admin instead.

This could be considered a major change and should maybe only be included in Solidus 3.0.0.

[jacobherrington]

This has the potential to be a really big change, but I'd love to see it made.

[patleb]

Changing solidus_frontend for solidus_storefront might help also.

[kingsleywang2013]

@kennyadsl I am happy to pull a request for this change. Did you start to implement it now? If not, i can do this part.

[kennyadsl]

@kingsleywang2013 sure, feel free to submit a PR for this, I've just added this task to the 3.0 Milestone after talking about that with the rest of the Core Team, thanks!

[trottomv]

I have the code ready for this issue, can I pull a request?

[jacobherrington]

@trottomv Please do!