Set a fixed date for frontend fixtures

What does this MR do?

Set a fixed date to use when generating frontend fixtures to make sure they only differ when something has actually changed.

$ grep -orE --color=auto '[0-9-]{10}T[0-9:.]{12}Z"' spec/javascripts/fixtures/**/*.json

Before

After

spec/javascripts/fixtures/deploy_keys/keys.json:2018-10-29T21:37:04.455Z" 2018-10-29T21:37:04.455Z" 2018-10-29T21:37:04.294Z" 2018-10-29T21:37:04.294Z" spec/javascripts/fixtures/merge_request_diffs/inline_changes_tab_with_comments.json:2018-10-29T21:37:26.541Z" 2018-10-29T21:37:26.541Z" spec/javascripts/fixtures/merge_request_diffs/parallel_changes_tab_with_comments.json:2018-10-29T21:37:28.059Z" 2018-10-29T21:37:28.059Z" spec/javascripts/fixtures/merge_request_diffs/with_commit.json:2016-09-27T14:37:46.000Z" 2016-09-27T14:37:46.000Z" 2016-09-27T14:37:46.000Z" 2018-10-29T21:37:25.661Z" spec/javascripts/fixtures/merge_requests/diff_discussion.json:2018-10-29T21:37:23.581Z" 2018-10-29T21:37:23.581Z" spec/javascripts/fixtures/merge_requests/discussions.json:2018-10-29T21:37:22.639Z" 2018-10-29T21:37:22.639Z" spec/javascripts/fixtures/merge_requests/image_diff_discussion.json:2018-10-29T21:37:25.040Z" 2018-10-29T21:37:25.040Z" spec/javascripts/fixtures/merge_requests/resolved_diff_discussion.json:2018-10-29T21:37:24.309Z" 2018-10-29T21:37:24.309Z" 2018-10-29T21:37:24.326Z" 2018-10-29T21:37:24.326Z" 2018-10-29T21:37:24.272Z" spec/javascripts/fixtures/pipelines/pipelines.json:2018-10-29T21:37:32.607Z" 2018-10-29T21:37:32.607Z" 2018-10-29T21:37:32.564Z" 2018-10-29T21:37:32.564Z" 2015-01-10T21:23:29.000Z" 2015-01-10T21:23:29.000Z" 2015-01-10T21:23:29.000Z" 2018-10-29T21:37:32.520Z" 2018-10-29T21:37:32.520Z" 2014-02-27T08:57:31.000Z" 2014-02-27T08:57:31.000Z" 2014-02-27T08:57:31.000Z"

spec/javascripts/fixtures/deploy_keys/keys.json:2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" spec/javascripts/fixtures/merge_request_diffs/inline_changes_tab_with_comments.json:2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" spec/javascripts/fixtures/merge_request_diffs/parallel_changes_tab_with_comments.json:2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" spec/javascripts/fixtures/merge_request_diffs/with_commit.json:2016-09-27T14:37:46.000Z" 2016-09-27T14:37:46.000Z" 2016-09-27T14:37:46.000Z" 2015-07-03T10:00:00.000Z" spec/javascripts/fixtures/merge_requests/diff_discussion.json:2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" spec/javascripts/fixtures/merge_requests/discussions.json:2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" spec/javascripts/fixtures/merge_requests/image_diff_discussion.json:2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" spec/javascripts/fixtures/merge_requests/resolved_diff_discussion.json:2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" spec/javascripts/fixtures/pipelines/pipelines.json:2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" 2015-01-10T21:23:29.000Z" 2015-01-10T21:23:29.000Z" 2015-01-10T21:23:29.000Z" 2015-07-03T10:00:00.000Z" 2015-07-03T10:00:00.000Z" 2014-02-27T08:57:31.000Z" 2014-02-27T08:57:31.000Z" 2014-02-27T08:57:31.000Z"

What are the relevant issue numbers?

part of https://gitlab.com/gitlab-org/gitlab-ce/issues/49127

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.