Support creating a template from an existing campaign

[ajohn25]

Is your feature request related to a problem? Please describe.
Users sometimes only know they want to create a template after an existing campaign has been sent out. Currently, creating a template from the campaign means manually typing in all the campaign details again.

Describe the solution you'd like
Mirror the workflow for creating a template from a campaign in the opposite direction:

1. Expand the + button on the templates page to add "Create Blank" and "Create from Existing Campaign" buttons
2. Add a button to the campaign stats page to "Convert to template"

Describe alternatives you've considered
N/A