Clarify roles of admins and contributors in Concordances

[mroehl86]

The roles of admins and contributors in a Concordance should be explained more clearly:

Admins

• can edit the concordance metadata
• can add and remove contributors

Important:
Being an admin does not mean you can edit mappings.
Admins must also be listed as contributors to work with mappings.

Contributors

• can add mappings
• can edit mappings
• can delete mappings

Important:
Contributors can only edit and delete their own mappings.
They cannot edit mappings created by others, either within the concordance or in general. This requires separate permissions!