MVP: Only show media tab

Until we support showing documents.

ts/components/conversation/media-gallery/MediaGallery.tsx

@@ -95,12 +95,14 @@ export class MediaGallery extends React.Component<Props, State> {
             isSelected={selectedTab === 'media'}
             onSelect={this.handleTabSelect}
           />
+          {/*
           <Tab
             label="Documents"
             type="documents"
             isSelected={selectedTab === 'documents'}
             onSelect={this.handleTabSelect}
           />
+          */}
         </div>
         <div style={styles.attachmentsContainer}>{this.renderSections()}</div>
       </div>