* nit: replaced harcoded string constants with named constants
* acl: added management of ACL entries
* auth: implemented DefaultAuthorizer which uses ACLs if any entries are found in the system and falls back to LegacyAuthorizer if not
* cli: switch to DefaultAuthorizer when starting server
* cli: added ACL management
* server: refactored authenticator + added refresh
Authenticator is now an interface which also supports Refresh.
* authz: refactored authorizer to be an interface + added Refresh()
* server: refresh authentication and authorizer
* e2e tests for ACLs
* server: handling of SIGHUP to refresh authn/authz caches
* server: reorganized flags to specify auth options:
- removed '--allow-repository-users' - it's always on
- one of --without-password, --server-password or --random-password
can be specified to specify password for the UI user
- htpasswd-file - can be specified to provide password for UI or remote
users
* cli: moved 'kopia user' to 'kopia server user'
* server: allow all UI actions if no authenticator is set
* acl: removed priority until we have a better understood use case for it
* acl: added validation of allowed labels when adding ACL entries
* site: added docs for ACLs