datasette: Ability for plugins to collaborate when adding extra HTML to blocks in default templates

Sometimes a plugin may want to add content to an existing default template - for example datasette-search-all adds a new search box at the top of index.html. I also want datasette-upload-csvs to add a CTA on the database.html page: https://github.com/simonw/datasette-upload-csvs/issues/18

Currently plugins can do this by providing a new version of the index.html template - but if multiple plugins try to do that only one of them will succeed.

It would be better if there were known areas of those templates which plugins could add additional content to, such that multiple plugins can use the same spot.