Fix that codes cannot be created on the fly if there are codes of the same name elsewhere in the hierarchy

[tanius]

Currently, a code cannot be created on the fly via the Annotator.js overlay window if a code of that name exists at some other location in the author's code hierarchy.

Example:

1. Create a code "digital media → social media" (means, parent code "digital media", child code "social media).

2. Try to code a selection of text with a not-yet-existing code "social media" (means, a code on the root level, not selecting the "digital media → social media" code proposed in the Annotator.js dropdown.

Result: status "422 API request failed" with JSON response {"tag":["can't be blank"]}.

The expected result is that the code should just be created as a root-level code. It is alright that a code of that same name and created by the same author already exists somewhere else in the code hierarchy. The principle is, any code that has a different text representation in the Annotator.js dropdown than any existing code should be possible to create on the fly.