[Group Templates] Refactor/reuse/unify file templates implementation feature implementation

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(Weight: 3) [BE]: Refactor/reuse/unify file templates implementation feature implementation to make use of Template Repository setting at the group level and let the projects within the group or its subgroups inherit the issue, merge request, epic description templates from given Template Repository, POC: !46222 (merged) There are 3 implementations for fetching templates data on BE as well

• issuables_helper generates code for HAML
• templates_controller
  • used on edit issue page through an AJAX call
• templates REST API - used by file templates I think

Availability and Testing

Manual job package-and-qa will need to be run in the MR to ensure e2e tests do not break.