Work with Spree 4 & Rails 6

[imgarylai]

I'm sorry for sending this huge pull request. :( I originally just wanted to make this excellent gem work first, and didn't think much of what I have done. So, the commits look a little messy.)

However, I believe my works might help somebody. Because there are many good pull requests, and I just organize some of them to make this gem work in a shot time! Since I'm gonna move forward to do other things, I hope someone could help me finish the rest of the things.

What has been done in this pull request?

• Update & fix configuration to the latest Spree convention
• Rewrite a small piece of coffee code in javascript (since coffee-script is not a default gem for most of new Rails app)
• Add UTF-8 support (I basically work on Chinese, but other languages like Japanese should work as well)
• Remove deprecated methods
• Fix most of the tests (I leave features test because I'm not familiar with Capabyra.)
• Update UI to the latest backend theme
• Fix rails 6 zeitwerk problem
• Update prawn-rails to ~> 1
• Remove the store path (local storage doesn't fit for most of the production cases. But, I'm working on saving files to Active Storage, it can help both on development and production. It might be in the next pull request)

If this PR looks useful to you, let me know if I can do anything to make this pull request to be merged.

Thank you.

[damianlegawiec]

Hey @imgarylai & @voleoo could you rebase this pull request? Thank you both for you excellent work so far! 💪

[imgarylai]

@damianlegawiec Yeah no problem.

[imgarylai]

@damianlegawiec I think I will create some small PRs to finish reset of the job in this PR because I changed too many things in this PR and some of them might not be necessary.

Let me close this issue first.