Make it clear to users they're creating a new dashboard from save chart modal

Currently, when the user is adding a chart to a dashboard from Explore, they can use a typeahead field to locate the dashboard to which they would like to add a chart.

However, when no dashboard exists and they will be creating a new dashboard, it is unclear that is what they will be doing.

Add "Create" cue in the typeahead field to make it clear to users they are creating a new dashboard.