Allow to find epics by the given time frame

On the epics roadmap page (see #3559) we need to display epics
in the given time frame. This patch adds an additional filter to
the Epic finder which allows to filter only epics which overlap
with the given time frame, epics which do not have start and end date
set are intentionally ignored.

This filter is not used anywhere yet, but it will be used in the MR for #3559.
This patch is a separate MR to avoid overgrowing and mixing too many
things together in the frontend patch for #3559.

Related #3559
91 jobs for ee-epics-by-timeframe in 39 minutes (queued for 11 seconds)
Status Job ID Name Coverage
  Build
manual #50398468
gitlab-org allowed to fail manual
package-qa
manual #50398469
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review-docs-deploy
 
  Prepare
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  Test
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  Post Test
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  Post Cleanup
manual #50398573
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review-docs-cleanup
 
Name Stage Failure
failed
flaky-examples-check Post Test
    "description": "Raw artifact when visiting old URL redirects to new URL",
"last_attempts_count": 2,
"flaky_reports": 1,
"first_flaky_at": "2018-01-31 15:01:59 +0000",
"last_flaky_at": "2018-01-31 15:01:59 +0000",
"last_flaky_job": "https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/50398534"
}
}
ERROR: Job failed: exit code 1