Upgrade to rails 6

[gryaznov]

Upgrade to the latest version of Rails and dependencies.

[theinventor]

Cool! It's not easy for me to tell from reading this, but,

1. does it leave out actioncable and some of the new stuff we aren't using?
2. did you manually verify in the web UI that the spreadsheet js stuff still renders/works? I bet we had no test coverage there, you should be able to drag in a spreadsheet, submit, and see it fire off api calls

[gryaznov]

@theinventor , hi!

1. I mentioned in the Slack channel that as one of the further steps it would be nice to revisit dependencies and clean them up. This includes explicit gems in Gemfile and those are under the hood (like actionmailer, actioncable etc.). I have not done this because of lack of time and because I was not sure about them and their possible role in the application (e.g. action mailer and coffee-rails were there for all this time but maybe this makes sense for some purposes unknown to me).

2. Please take a look here:

Started PATCH "/imports/8cbc6f4b-d92a-4f62-a379-8a6499aa0083" for ::1 at 2020-06-04 20:44:00 +0300
Processing by ImportsController#update as JS
  Parameters: {"import"=>{"platform"=>"syncro", "subdomain"=>"", "api_key"=>"123123123", "rows_to_process"=>"", "errors_to_allow"=>"", "match_on_asset_serial"=>"0", "staging_run"=>"1", "mapping"=>"DD/MM/YYYY", "data"=>"[FILTERED]"}, "commit"=>"Process Update", "id"=>"8cbc6f4b-d92a-4f62-a379-8a6499aa0083"}
  Import Load (0.2ms)  SELECT "imports".* FROM "imports" WHERE "imports"."uuid" = $1 LIMIT $2  [["uuid", "8cbc6f4b-d92a-4f62-a379-8a6499aa0083"], ["LIMIT", 1]]
  Rendering imports/update.js.erb
  Rendered imports/update.js.erb (Duration: 0.4ms | Allocations: 313)
Completed 200 OK in 3ms (Views: 0.7ms | ActiveRecord: 0.2ms | Allocations: 2621)

Well, I am not sure about the last part but if there is something wrong or missing, please, tell me and I will make appropriate fixes/changes.

[stevenjonescgm]

Regarding merging this for https://repairtechsolutions.atlassian.net/browse/RS-10919

Looks like this removes mimemagic and so rails 6.0.3.6 is sufficient.

So do we need to have QA, or perhaps just coordinate with shipping and testing in production if rollback required.