Show Export Metadata only for the latest released dataset version

[ChengShi-1]

What this PR does / why we need it:

• Show "Export Metadata" button on file pages, only for the latest released dataset version
• update "isLatest" param check if the current ver is latest

Which issue(s) this PR closes:

• Closes The "Export Metadata" button is misleading when viewing an older version of a dataset because it gives you the latest published version #1008 Feature Request/Idea: Export metadata for drafts #1007

Special notes for your reviewer:

• isLatest(of published version) doesn't exist in api result, so I did it by comparing current version to getVersionSummaries

Suggestions on how to test this:

• create a dataset and upload file
• update some metadata and publish so there are multiple versions of this dataset
• check in file version, the "Export Metadata" button on dataset pages and file pages, only show for the latest released dataset version, and hide for other versions
• Also, for a draft version, as a dataset owner, the button should show and return draft version metadata

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

Is there a release notes or changelog update needed for this change?:

yes

Additional documentation:

[coveralls]

coverage: 97.295% (-0.05%) from 97.343% — 1008-the-export-metadata-button-is-misleading-when-viewing-an-older-version-of-a-dataset-because-it-gives-you-the-latest-published-version into develop

[pdurbin]

Would it make sense to address the following issue in the same PR?

• Feature Request/Idea: Export metadata for drafts #1007