A tag name must be unique and can't be used as a category. There were no error message when creating a tag identical to an existing category. Now, this is addressed.

See #7686

Closes #7686

Changes proposed in this pull request:

• add validation on tag creation

How to test the feature manually:

1. create a new category (ex: HW)
2. create a new tag with the same name as the new category (ex: HW)
3. validate that the appropriate error message is displayed

Pull request checklist:

• clear commit messages
• code manually tested
• unit tests written (optional if too hard)
• documentation updated

Additional information can be found in the documentation.