Avoid N+1 queries in epic reference

What does this MR do?

Optimizes epics reference method and avoids N+1 queries in epic's autocomplete.

Related to #331118 (closed)

DB group preload query:

SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."type" = 'Group' AND "namespaces"."id" IN (5754519, 7137538, 5032027, 2351283, 2750817, 2593421, 6104555, 2639717, 7983076, 6891292)

Index Scan using index_namespaces_on_type_and_id_partial on public.namespaces  (cost=0.43..21.58 rows=2 width=346) (actual time=26.439..103.688 rows=10 loops=1)
   Index Cond: (((namespaces.type)::text = 'Group'::text) AND (namespaces.id = ANY ('{5754519,7137538,5032027,2351283,2750817,2593421,6104555,2639717,7983076,6891292}'::integer[])))
   Buffers: shared hit=12 read=32
   I/O Timings: read=103.273 write=0.000

Screenshots (strongly suggested)

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