Improve display of included/excluded recipients in CiviMail creation

In previous versions of Civi (and in the current version of the SMS creation tool) the groups to 'include' and 'exclude' in a mailing were in separate input boxes.

Recently this was combined into a single field:

This has pros and cons! Our communications team has highlighted some shortcomings with this approach.

Pros:

• One field to rule them all, simpler UI

Cons:

• Performance (handled separately in CRM-18550 - though it has improved!)
• Creates potential for user error when groups are 'included' rather than 'excluded' - this can have bad consequences (we know from experience) - since both are in the 'recipients' box
• The styling is not good for accessibility - it's important to be able to clearly read the name of exluded groups - but it's got a strikethrough! We can't just use colour to differentiate it, because that's no good for colour blindness.

Proposed options

Option 1) Split the field back out
Have a field for 'Recipients' and a separate field for 'Excluded recipients'. This would make the separation more obvious, and this would also improve the performance of the fields. (If 'excluding' recipients is a less common task then we could hide/collapse the input by default?)

Option 2) Make the existing field much clearer
Through improved styling/display only?

Next steps

Discuss! We may be able to fund/support the development for this, but would like some community feedback first.