Add global JavaScript user callbacks

Idea: pheix-io/ethelia#29 (closed)

Issues to be sorted out:

1. Requirements for UI/UX widgets, minimal list:
   • Specific id for block to be hidden on login/auth (BLOCK_A);
   • Specific id for block to be shown on login/auth (BLOCK_B);
   • Riot.js component for BLOCK_B,  shall we render it? 
2. Distinguish login/auth, logout/exit, extend or refresh events in callbacks;
   • on refresh we should update fallback to BLOCK_A and then revert BLOCK_B on successful refresh.
3. User's JavaScript code integration, callback names, arguments;