Moved the Material Link Management to the Event Edit Interface

Reworked how material links are handled using nested forms with the
cocoon gem. The list of material link forms on the event edit form are
now dynamic. Still have some code cleanup to do along with some styling
tweaks.
More Code Cleanup from Previous Material Links Implementation
material form styles
More Material Links Code Cleanup
Merge branch 'move_material_links' of https://github.com/extension/learn into move_material_links
fixing conflicts
Added Event Materials Capability to Conference Events
Add Evaluation Link Option to Conference Events
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