fix: assign tags created from the project dialog

The tag dialog's form is nested inside the project dialog's form in the
React tree, and React propagates events along the React tree rather than
the DOM tree, so submitting the tag form also submitted the project form.
The project was created immediately with no tags and the dialog closed
before more than one tag could be picked.

Stop propagation in the tag form's onSubmit, and select the new tag in
the selector so it is included when the project is saved.

Fixes #5117