Fix icon size type

[danxuliu]

[Vue warn]: Invalid prop: type check failed for prop "size". Expected Number with value 16, got String with value "16".

found in

---> <CloseIcon> at node_modules/vue-material-design-icons/Close.vue
       <ButtonVue>
         <ContactSelectionBubble> at src/components/LeftSidebar/NewGroupConversation/SetContacts/ContactSelectionBubble/ContactSelectionBubble.vue
           <TransitionGroup>
             <SetContacts> at src/components/LeftSidebar/NewGroupConversation/SetContacts/SetContacts.vue
               <Modal>
                 <NewGroupConversation> at src/components/LeftSidebar/NewGroupConversation/NewGroupConversation.vue
                   <AppNavigation>
                     <LeftSidebar> at src/components/LeftSidebar/LeftSidebar.vue
                       <Content>
                         <App> at src/App.vue
                           <Root>

I have not tested the Matterbridge icon, but I guess it has the same problem.