Increasing z-index of Select component to overlay other elements.

Before this change, other elements (namely react-bootstrap input
components with addons) overlapped with `Select` inputs due to a higher
`z-index` of the first.

This change is increasing the `z-index` of the `Select`/`react-select`
menu to `3` (react-bootstrap input addons are on `2`) to prevent this.

Fixes #4769.

graylog2-web-interface/src/components/common/Select.css

@@ -8,4 +8,9 @@
 
 .select-sm .Select-input > input {
     padding: 0;
-}
+}
+
+:local(.increaseZIndex) .Select-menu-outer {
+    z-index: 3;
+}
+

graylog2-web-interface/src/components/common/Select.jsx

@@ -165,7 +165,7 @@ const Select = createReactClass({
     }
 
     return (
-      <div className={size === 'small' ? 'select-sm' : ''}>
+      <div className={`${size === 'small' ? 'select-sm' : ''} ${this.reactSelectSmStyles.locals.increaseZIndex}`}>
         <SelectComponent ref={(c) => { this._select = c; }}
                          onChange={onReactSelectChange || this._onChange}
                          {...reactSelectProps}