===Background
Large templates and multi-part template content with many templates can take a very long time to load and in usability tests users thought that something was broken, clicking on the edit button many times.
This is the follow up to the investigation: {T296386}
===Requirements
[] Open dialog immediately after user clicks on edit or double clicks on an existing template
[] When content needs time to load, show the header and blank overlay with OOUI widget Progress bar (indeterminate). Replace overlay with template content when it's fully loaded.
===Nice to have
[] The overlay is 90% opacity and shows the elements behind as they load
===Mocks and specs
Progress bar is 70% width of the dialog and is centered in both directions
100% Opacity:{F34766700}
Specs:{F34766699}
90% Opacity (Nice to have): {F34766701}