Allow users to cull subject from subject page

[k1o0]

To make it easier for user to enter cull information for a given subject we can make the following changes:

• Modify outcomes inline such that cull method, actual severity, and adverse effects are drop downs or multi-select boxes instead of free text
• Allow user to add a cull and a death date here
• Validate form such that actual severity cannot be blank
• Remove cull method field from subject model (requires migration)

This is related but different to the stock manager requirements of culling many mice at once via the cull_subjects page.