Add mixin change request to Dashboard

[schollz]

On the dashboard.tmpl I would like to add another Well. This Well appears when a user clicks on "Mixin = X.Y" and features a a JQuery slider and two buttons. The JQuery slider can be set to a value between 0 and 1. The two buttons should be labeled "Default" and "Override" or similar.

The idea is that you can set the mixin variable, manually, to override the one currently set by the computer. When "Default" is set, the dashboard should make a PUT request to set the mixin to be -1, whereas when "Override" is clicked, the dashboard should make a PUT request to set the mixin to be whatever the slider value is.