Remove full-stack Rails as a dependency

[nimmolo]

I can't tell what parts of Rails this gem depends on, but mission_control-jobs does not appear to depend on the whole Rails stack.

#I would like to try the gem in production, but our app does not load ActiveStorage, ActionText or ActionMailbox. I think it's pretty common for people to just use some Rails gems. This may also relate to API-only apps (#60).

Other gem examples:

• Solid Queue PR #158
• Solid Cache gemspec

PR started at #121. Is this possible currently?