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

**Feature**: Dropdown menus to add assignee and milestone from the issue#show page.

![screen shot 2013-12-15 at 6 47 03 pm](https://f.cloud.github.com/assets/1238532/1751416/176f5fa8-65e5-11e3-9f95-4f3982718bc8.png)

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 :)