Auth refactor, phase 1

This is an initial phase of refactoring salesforce module's auth framework:
 * new Salesforce Auth Config entity to allow multiple auth configs per site, for better environment management
 * introduce lusitanian/oauth and firebase/php-jwt composer requirements
 * create new services to wrap token and config management
 * glue code for backwards compatibility and non-disruptive changes

Squashed commit of the following:

commit 432d4a93
Author: Aaron Bauman <aaron@messageagency.com>
Date:   Fri Dec 28 16:45:28 2018 -0500

    Couple more updates to docs

commit 1b3d8ed9
Author: Aaron Bauman <aaron@messageagency.com>
Date:   Fri Dec 28 16:39:57 2018 -0500

    Code sniff cleanup

commit 27ce985d
Merge: e9772c3e 21e69b44
Author: Aaron Bauman <aaron@messageagency.com>
Date:   Fri Dec 28 15:20:21 2018 -0500

    Merge branch '8.x-3.x' of bitbucket.org:messageagency/sf into auth_refactor_phase1

commit e9772c3e
Merge: 65cbc208 37dd412f
Author: Aaron Bauman <aaron@messageagency.com>
Date:   Fri Nov 30 15:39:47 2018 -0500

    Merge branch '8.x-3.x' of github.com:messageagency/sfd8 into auth_refactor_phase1

commit 65cbc208
Author: Aaron Bauman <aaron@messageagency.com>
Date:   Thu Nov 15 16:30:09 2018 -0500

    Update hook to update acess tokens

commit b70506a5
Author: Aaron Bauman <aaron@messageagency.com>
Date:   Thu Nov 15 15:48:43 2018 -0500

    Auth refactor, refactored. Phase 1 of ?