add config to allow TAs to set API keys

[davewhiteland]

Currently Teaching Assistants can't set API keys on the /admin/api-keys page... but can set their own API key (via a button on the API test page, which TAs do have access to).

Fix this by

• adding a config setting IS_TA_SET_API_KEY_ENABLED
  (similarly to existing TA config settings IS_TA_EDIT_COMMENT_ENABLED & IS_TA_PASSWORD_CHANGE_ENABLED)
• adding/removing/disabling API key buttons for TAs (e.g., that one on the test page)
• checking TA access in general: e.g., should probably have access to Set-up Summary

Related to the work on re-organising the admin buttons #215

[davewhiteland]

closed by 18f9c65fdaecf0a7b5fbeb5ea889252eacdde52f