Upgrade abraham to use latest version of Shepherd.js

[jabbett]

Abraham was developed when Shepherd.js was at version 1.8... it's now at 5.0 (and no longer maintained by HubSpot, to boot).

According to https://github.com/shipshapecode/shepherd/issues/624, we should upgrade to the v6 beta since that's being released soon.

[jabbett]

• [x] Revise package.json
• [ ] Update README
• [x] Update generated JS in _abraham.html.erb
• [x] Make full tour_options configurable from abraham.yml
• [x] Make the old Shepherd themes available for legacy users