ANW-699: moving 'add item' button to bottom for defined lists

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Description

Ease of use, so that adding more items doesn't require scrolling up so much.

Related JIRA Ticket or GitHub Issue

https://archivesspace.atlassian.net/browse/ANW-699

How Has This Been Tested?

manual in browser testing

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.
• I have authority to submit this code.
• I have added tests to cover my changes.
• All new and existing tests passed.