Added dropdown menus to issue#show page for assignee and milestone

Created by: jasonblanchard

Feature: Dropdown menus to add assignee and milestone from the issue#show page.

This feels like something I should be able to do easily without editing other parts of the issue to speed up the ticket workflow.

Implementation: Added form on issue#show with select fields for assignee and milestone. This form will submit remote to the issue#update action. JS response re-renders the issue_context partial based on the params[:issue_context] value. The form includes a submit button for graceful degradation and testing.

I'm not sure this is the right placement for these dropdown menus, so I'm definitely open to suggestions here :)