Bug 35919: Add record sources admin page

Bug 35919: Add record sources admin page

This patch introduces a Vue.js based record sources managing page. To
test it:

1. Apply this patch
2. Build the Vue.js stuff:
   $ ktd --shell
  k$ yarn js:build
  k$ restart_all
3. On the staff interface, go to Administration > Record sources
4. Play with the interface and the offered actions
=> SUCCESS: Things go well
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>