Bug 28772: Display API secret once

Bug 28772: Display API secret once

This patch makes the apikeys.pl display the generated API secret once,
when generated. After that, it won't be displayed by the UI.

To test:
1. Generate a new API key
=> FAIL: The secret is displayed in the API keys table
2. Visit some other page, and go back to the API keys page
=> FAIL: The API key secret is there
3. Apply this patch
4. Go to More > Manage API keys
=> SUCCESS: It no longer displays the secret
5. Generate a new API key
=> SUCCESS: The API key details (including the secret) are displayed.
=> SUCCESS: A message telling to copy the secret because it won't be
            displayed again is shown.
6. Repeat 4
=> SUCCESS: The secret is no longer displayed
7. Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>