Proposal: Upgrade to rails 6 + authentication scheme

[JensDebergh]

Hey there!

I was wondering if you guys would be open to accept a pull request which does following things:

Proposal:

• Upgrade to rails 6

Reason:

I'm using rails 6 and would like to use lines engine as it solves most of my problems, but I'm unable to since it is rails 5 only.

Proposal:

• Step away from the predefined authentication scheme like devise you are using.

I don't mean directly removing devise as a whole but make it configurable by the user. Be it http authentication or custom authentication.

Reason

It seems unnecessary to bring in a complete new authentication scheme / dependencies + lines_user table if I already have a user table in my database.

I would propose to add a function call that lines calls when checking for authenticated users.

# Authentication scheme is handled and determined by the developer that uses the engine
authenticate -> { |username, password|  return true | false } 

Kind regards
Jens