[Front Office] Add a possibility to edit and remove risk owners

[jfrocha]

Implement risk owners management.

[ruslanbaidan]

It can be managed on the risks edit page, but will have limitations:

• to edit or remove we need to first search for a risk owner by name
• if the risk owner is linked to any other risk it should be forbidden.

It might be more clear to add the functionality as a separate tab in the Knowledge Base. The validation can be performed and notice shown if the Risk owner is already linked to some risk(s) in case of removal attempt.
The separate page of Risk Owners management can include import them from a file (requested #411).