appview/repo: add ?intent=star,fork dialog on repo pages

Adds support for `?intent=star` and `?intent=fork` on a repository page. External sites can now drop a "Star on Tangled" or "Fork on Tangled" button that links to e.g. `tangled.org/alexisbouchez.com/hypercommit?intent=star`, and the repo page opens a small confirmation dialog with a primary action button. Unauthenticated visitors are sent through `/login` and land back on the dialog after. Closes [#498](https://tangled.org/tangled.org/core/issues/498).