Fix zindex on react-datepicker popover (#3376)

Small one-liner. Because EuiDatePicker doesn't actually use EuiPopover (which uses portals), but instead mimics their look, they need to carry a z-index to them in case they compete with other Dom sibling level components that might have a z-index (like EuiDataGrid, which uses it for focus).

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## [`master`](https://github.com/elastic/eui/tree/master)
 
+- Applied `max-width: 100%` to `EuiPageBody` so inner flex-based items don't overflow their containers  ([#3375](https://github.com/elastic/eui/pull/3375))
+
+**Bug Fixes**
+
+- Fixed z-index issue in `EuiDatePicker` where it's popover would sit beneath other DOM siblings that had z-index applied ([#3376](https://github.com/elastic/eui/pull/3376))
 - Added `download` glyph to `EuiIcon` ([#3364](https://github.com/elastic/eui/pull/3364))
 - Applies `max-width: 100%` to `EuiPageBody` so inner flex-based items don't overflow their containers  ([#3375](https://github.com/elastic/eui/pull/3375))
 - Added `ReactElement` to `EuiCard` `image` prop type to allow custom component ([#3370](https://github.com/elastic/eui/pull/3370))

src/components/date_picker/_date_picker.scss

@@ -109,7 +109,7 @@
 // }
 
 .euiDatePicker.euiDatePicker--shadow .react-datepicker-popper {
-  z-index: 1;
+  z-index: $euiZContentMenu;
   animation: euiAnimFadeIn $euiAnimSpeedFast ease-in;
 
   &[data-placement^="bottom"] {