Re-add custom icons for Application templates

How: Functionality used to exist to use custom icons for application templates. We would like to see this feature return.

Why: When multiple application templates are introduced in an Inriver instance, icons could help users distinguish between the templates, at a glance.

Additional context: Tis feature used to exist with the old Inriver UI, not sure why it was removed.

No category exists for HtmlTemplates, so this has been placed under extensions.

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