Add validate only check in the Import Metadata page

[atarix83]

References

Add references/links to any related issues or PRs. These may include:

• Fixes Import Metadata CSV (Enhanced, with Preview of Changes, as in v6.x) #782

Description

This PR add the --validate-only flag to the metadata import page

Instructions for Reviewers

As admin open the metadata-import page (admin/metadata-import) and check the new checkbox works properly

Checklist

• My PR is small in size (e.g. less than 1,000 lines of code, not including comments & specs/tests), or I have provided reasons as to why that's not possible.
• My PR passes TSLint validation using yarn run lint
• My PR doesn't introduce circular dependencies
• My PR includes TypeDoc comments for all new (or modified) public methods and classes. It also includes TypeDoc for large or complex private methods.
• My PR passes all specs/tests and includes new/updated specs or tests based on the Code Testing Guide.
• If my PR includes new, third-party dependencies (in package.json), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.

[tdonohue]

@atarix83 : Thanks! I tested this and it works exactly as described. I just have one small comment.

I think we should add a "hint" (formatted like hints in the submission form) under the "Validate Only" checkbox to describe its behavior.

[tdonohue]

👍 Thanks @atarix83 ! This looks good to me now. Re-tested today and everything is working well.