FE: Create filter for 'Source'

Why are we doing this work

• with inherited policies showing up on the policies list, a user will want to filter the policies by source

Relevant links

Non-functional requirements

• Documentation:
• Testing:

Implementation plan

• frontend create a policy source filter at ee/app/assets/javascripts/threat_monitoring/components/policies/filters/policy_source_filter
• frontend move policy_type_filter to ee/app/assets/javascripts/threat_monitoring/components/policies/filters/policy_type_filter
• frontend move policy_environments to ee/app/assets/javascripts/threat_monitoring/components/policies/policy_environments
• frontend modify policies_list.vue to show a the policy source filter