Merge branch 'ie-browser-update' into 'master'

Changes support from IE 10+ to IE 11+

IE 11 was released 17 October 2013; 2 years ago. Generally what I have experienced is that support for browsers (when newer ones are available) continues for up to 2 years. Based on that info we would drop support for IE 10 and the latest IE we would support would be IE 11. 

I also think this is more realistic since no one is really testing anything on IE 10 at the moment.  We should start testing on all the browser that we currently support. 

Also we can then have real support for flexbox and not need things like `-ms-flexbox` and other IE specific hacks. 

cc @iamphill @annabeldunstone @elvongray @alfredo1 @rspeicher 

See merge request !3913

doc/install/requirements.md

@@ -142,4 +142,4 @@ On a very active server (10,000 active users) the Sidekiq process can use 1GB+ o
 - Firefox (Latest released version and [latest ESR version](https://www.mozilla.org/en-US/firefox/organizations/))
 - Safari 7+ (known problem: required fields in html5 do not work)
 - Opera (Latest released version)
-- Internet Explorer (IE) 10+ but please make sure that you have the `Compatibility View` mode disabled.
+- Internet Explorer (IE) 11+ but please make sure that you have the `Compatibility View` mode disabled.