Bug 20624: Make staff client respect RESTOAuth2ClientCredentials

Bug 20624: Make staff client respect RESTOAuth2ClientCredentials

This patch makes the staff client UI respect the
RESTOAuth2ClientCredentials syspref.

To test:
- Make sure RESTOAuth2ClientCredentials is "Don't enable"
- Go to a patron's detail page
=> SUCCESS: The 'More' dropdown doesn't show the API keys management
link.
- Enable RESTOAuth2ClientCredentials
- Reload
=> SUCCESS: The 'More' dropdown shows the API keys management link
- Click on the API keys management link
=> SUCCESS: You can edit the api keys
- Disable the syspref
- Reload
=> SUCCESS: You are presented an error 400 page.
- Sign off :-D

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>