[rotang] A lit-element used to create/modify rotation configurations.

https://rota-ng-staging.googleplex.com/managerota - The manage button and the "Create new rotation" link uses the element.

Bug: 845549
Change-Id: I7647408b80d1cb86f8069a3c2ce897e27dcf4aea
Reviewed-on: https://chromium-review.googlesource.com/c/1351859
Commit-Queue: Ola Karlsson <olakar@chromium.org>
Reviewed-by: Tiffany Zhang <zhangtiff@chromium.org>
Cr-Commit-Position: refs/heads/master@{#19289}