Fix scan execution policy selection

What does this MR do?

This cleans up the policy selection handling by directly storing the selected policy object instead of computing it based on the policy' name. This fixes a bug where the policy drawer would remain empty when selecting a scan execution policy as the selectedPolicy object was being computed based on networkPolicies only.

This also fixes a bug where editing a network policy would lead to an error 404. Including the policy kind in the edit URL should resolve the issue as the backend now expects this parameter to load policy data to be passed to the frontend.

Screenshots or Screencasts (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?) Changelog is not required because it's not yet possible to create scan execution policies, unless the security_orchestration_policies_configuration feature flag is enabled.
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.