fix: firefox event click bug (jquense#1262)

I've faced the same problem as @Kerumen described in jquense#1173 as it was long time ago, I've decided to push pr with fix

Author: michalak111

src/EventCell.js

@@ -51,8 +51,9 @@ class EventCell extends React.Component {
 
     return (
       <EventWrapper {...this.props} type="date">
-        <button
+        <div
           {...props}
+          tabIndex={0}
           style={{ ...userProps.style, ...style }}
           className={cn('rbc-event', className, userProps.className, {
             'rbc-selected': selected,
@@ -64,7 +65,7 @@ class EventCell extends React.Component {
           onDoubleClick={e => onDoubleClick && onDoubleClick(event, e)}
         >
           {typeof children === 'function' ? children(content) : content}
-        </button>
+        </div>
       </EventWrapper>
     )
   }

src/less/event.less

@@ -1,17 +1,11 @@
 @import './variables.less';
 
 .rbc-event {
-  border: none;
-  box-shadow: none;
-  margin: 0;
   padding: @event-padding;
   background-color: @event-bg;
   border-radius: @event-border-radius;
   color: @event-color;
   cursor: pointer;
-  width: 100%;
-  text-align: left;
-
   .rbc-slot-selecting & {
     cursor: inherit;
     pointer-events: none;