Refactor Component and secrets

Merges #24 (closed) and #19 (closed). Since the only component is Worker in recent development, Component shall be renamed to Worker and have the following parameters added: url (for accessing the worker), tags for storing tags representing the Worker's capabilities, and STATE (CREATED, READY, STOPPED).

A new entity shall be created (Client) and be bound to Role in place of User. Both Users and Workers shall be Clients and as such use the same permissions system. It should be noted that this change increases the importance of correct Permissions configuration in a role (and sensible defaults). The new entity shall contain an attribute type which can have values of user or worker (others could be added later).

Each Client shall contain a list of Secrets which may be used for authentication (mostly CLI client and Workers).

Affected parts of Kontr portal and frontend: db, permissions handling in rest, backend tests, frontend entities (more?)