Add 'Assign system tags' permission to limit system tag selecting

Problem/Motivation

Anyone with permission to edit nodes where an entity reference field is added to select system_tag entities, can update this field.
This module is more of a 'power user' module in a way that a user with limited knowledge can break the system tags.

Steps to reproduce

1. Add entity_reference field targeting system_tag entities to a content type.
2. Create a node of this type as a non-admin user.

Proposed resolution

• Add 'Assign system tags' permission.
• Use this permission to shield access to entity_reference fields targeting system_tag entities.

Remaining tasks

?

User interface changes

N/A

API changes

N/A

Data model changes

N/A