Add scheduled_at field to jira_imports table

What does this MR do?

#213808 (closed)

Adds the field scheduled_at to the table jira_imports. This field then is updated when the import job is scheduled. Also, it uses this field in JiraImportType instead of created_at.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Migration outputs

Table creation (20200423101529)

UP

bundle exec rails db:migrate:down VERSION=20200423101529
== 20200423101529 AddScheduledAtToJiraImports: reverting ======================
-- remove_column(:jira_imports, :scheduled_at, :datetime_with_timezone)
   -> 0.0024s
== 20200423101529 AddScheduledAtToJiraImports: reverted (0.0062s) =============

DOWN

bundle exec rails db:migrate:up VERSION=20200423101529
== 20200423101529 AddScheduledAtToJiraImports: migrating ======================
-- add_column(:jira_imports, :scheduled_at, :datetime_with_timezone)
   -> 0.0020s
== 20200423101529 AddScheduledAtToJiraImports: migrated (0.0021s) =============

Closes #213808 (closed)