Bug 29482: Replace the term branch with library

Bug 29482: Replace the term branch with library

This patch replaces the word "branch" with the word "library" in the
self-checkout template, as per terminology guidelines.

To test, apply the patch and go to Administration -> System preferences.

- Set the "IndependentBranches" preference to "Yes."
- Log into the self checkout system using a staff user with permission
  to do self-checkout but not superlibrarian status.
- Try to check out an item which belongs to a library that's different
  to the one you're logged in with.
- You should see a message, "Item cannot be checked out," including the
  text "This item belongs to another library."

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>