[BE] Correctly handle default template for new work items

From our documentation, the priority of description templates are:

1. Template set in project settings.
2. Default.md (case-insensitive) from the parent group.
3. Default.md (case-insensitive) from the project repository.

When we have all of these set and we create a new issue NOT using the create flow, the backend does not use the template in the project settings. Instead, it uses the template in the repository.

We possibly have to update the assign_description_from_template method so the backend uses the template in the project settings when it exists.

Acceptance criteria

• In the UI, when we create a new work item without the create flow (for example creating a new issue on a board, or creating a new child item on an epic) the new work item should have the description template from the projects settings set as its description if it exists. This should also be the case when submitting a WorkItemCreate mutation