Flexible layout templates: Allow selecting an existing layout template, as a starting point

This is part of #4046, and a sibling-issue to #4114.

As it is now, when one starts creating a flexible layout template, they are required to enter a name for it, and (optionally) a description. Once they do that and submit the form, they get a flexible layout template with 3 default single-region rows: header/content/footer. They can then start customizing rows and regions as they see fit.

What I am proposing here is that at the step where they are entering the template name, there is also a select list of existing templates (both standard ones, as well as flexible ones). The user can choose one of those, so after they submit the form, they end up with more rows/regions. Now, instead of having to add rows/regions from scratch, they have a good starting point ...less clicks -> less work to do -> better UX