Allow conditions plugins to be added to individual Conditional Features

Problem/Motivation

Conditional Features should utilise Conditions Plugin API so various conditions plugins can be applied to a conditional feature and the feature will show/hide on more than just the current enabled flag.

Remaining tasks

Get and add list of condition plugins to conditional feature form. Run evaluate method on each enabled condition plugin against condition feature.

Create a global conditional feature settings page to select which conditions plugins are available to individual conditional feature.

User interface changes

Update conditional feature add/edit form.

Create conditional features settings form.