Editing a context config overrides all context configs

Steps to reproduce:

1. Verify that the table scald_context_config is empty (truncate if necessary).
2. Go to /admin/structure/scald/image/contexts.
3. Make changes to a single context and save.
4. Check the table scald_context_config again.

Expected result:
Only an entry for the modified context has been created.

Actual result:
Entries have been created for all contexts.

Proposed solutions:

• Variant A: Verify that changes have been made to a context before saving its config object. I would not recommend this solution as I expect it to be rather fragile.
• Variant B: Change the UI so that contexts are edited individually. This is the least error prone solution and much more in line with other contrib modules that utilize ctools exportables.