[githubql] Update index patterns with MergedEvent and new study fields

[mafesan]

This PR includes the new fields from the MergedEvent items, added at https://github.com/chaoss/grimoirelab-elk/pull/947:

• merge_closed
• merge_closed_at
• merge_created_at
• merge_merged
• merge_merged_at
• merge_updated_at
• merge_url

It also includes the two new fields from the updated cross-references study, added at https://github.com/chaoss/grimoirelab-elk/pull/948:

• referenced_by_merged_prs
• referenced_by_external_merged_prs

The following index patterns have been also updated, as they include this two new fields too:

• github_issues-index-pattern.json
• github2_issues-index-pattern.json
• github2_pull_requests-index-pattern.json

[zhquan]

I miss the field duration_from_previous_event used by the dashboard GitHub Events - LabelEvents

[mafesan]

Thank you for the heads-up @zhquan, it should be fixed now. I have reviewed all of them just in case, so it is now ready for review again.