Add an optionnal description field to the label types

[michaelpixelscoder]

Adding an optionnal description field to the label types can help finding the right tag when there is a lot of tags.

Steps:

• backend: Add field to the backend model
• backend: Implement it in the serializers
• backend: Implement it in the views
• backend: Make sure import labels support description
• backend: Make sure demo projects are not broken
• backend: Test all types of labels
• frontend: Add the description field to all label base types
• frontend: Add the description field to the label creation form
• frontend: Add the description column to the label list in second position
• frontend: Make the search bar able to search by the text and description field
• frontend: Test all types of labels
• (backend: Add a search entry point to text search by text and description)
• frontend: Make LabelingMenu query labels based on input text. Search in text and description