Make Feeds default template configurable

I am changing the feeds template on all my feeds. It would be helpful if I could configure the default template both instance-wide and per room.

https://github.com/matrix-org/matrix-hookshot/issues/883 related and would be helpful to get at the same time to avoid needing to remove and re-add all feeds to unset the template override

Start by tracing how feed templates are selected and configured in the TypeScript codebase, and review the related issue #883 for the existing template-override behavior. Done means users can set a default feed template instance-wide and per room without removing and re-adding feeds.

Written by the indexing model from the issue text.