Add support for compound audiences

[roborourke]

This is a capability in Pinpoint, you can create audiences from a combination of other existing audiences.

We should be able to combine and list the audiences already created by the user. This list should be visible in the dropdown selection when selecting a new audience.

Given: I am trying to create a new audience When: I select an audience from the dropdown Then: The dropdown should also show a list of existing audience And: Allow selection of this pre-existing audiences created my me.

The option to select an existing audience should be listed under "Select a field" in the dropdown before any other sections. Thereafter, the options in the second dropdown should be disabled except "is" option. Let the user select an existing audience that will be listed under the third dropdown.