Improve policy validation

From &5362 (comment 665128157)

When the user clicks the "Create merge request" button, we should check to see if they have a least one branch defined. If not, then we should show the following error message.

Implementation

• frontend update DEFAULT_SCAN_EXECUTION_POLICY to the following:

type: scan_execution_policy
name: # name is mandatory
description: ''
enabled: false
rules:
  - type: pipeline
    branches:
      - # branch is mandatory
actions:
  - scan: dast
    site_profile: ''
    scanner_profile: ''

• frontend add name and branches validation on the yaml and (this will be done as part of Interactive validation in security policy editor (#341419 - closed)) show error if it does not show, per the design. Link to docs for more information. This should be done in scan_execution_policy_editor component
• backend Add a check to ProcessPolicyService to ensure that all branches exist on policy_configuration.security_policy_management_project.
• backend Add new test cases to process_policy_service_spec.rb to ensure that policies cannot be created for non-existent branches.

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