.toggle-layout pulse animation causes constant repaint loop and high cpu utilisation

Steps to reproduce:
1. go to http://akveo.com/ngx-admin/#/pages/ui-features/grid
note that this page is static, without any animated diagrams
2. run `top` and observe cpu utilisation (or run devtools/performance profiler)
3. open devtools and disable `animation` and `-webkit-animation` in `.toggle-layout i::after` (cogs icon in the top right corner)
4. observe cpu utilisation

Problem may not be apparent on a workstation.
If it doesn't look like a big deal try running this test on an average consumer laptop.

Tested in Chrome and Firefox.

May be connected to issue https://github.com/akveo/ngx-admin/issues/1873