Bug 21674: Prevent to insert twice the same library in a group

Bug 21674: Prevent to insert twice the same library in a group

When you create hierarchical groups of libraries you are not allowed to add
several times the same library to a given group/subgroup. The libraries
that are part of the group is not displayed on the interface, to prevent
the user to select it.

However this restriction is only done template-side, the controller and
module does not handle it.

To prevent that this patchset enforces the constraint at DB level, and
display a message to the end user.

Test plan:
- Go to Home › Administration › Library groups
- Add a group
- Add a library
- reload the page several times
=> Without this patch the same library appears several times in the group
=> With this patch applied you will see a friendly error message

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>