Gallery fixes (#514)

- Fix COC overlay height
- Make standalone gallery usable on mobile devices

Before:
![image](https://user-images.githubusercontent.com/693092/110415215-81cd0680-80b7-11eb-8000-bd0b8536607a.png)

After:
![image](https://user-images.githubusercontent.com/693092/110415236-898cab00-80b7-11eb-8266-94a5718113fe.png)

src/Explorer/Controls/NotebookGallery/GalleryViewerComponent.less

@@ -11,7 +11,7 @@
 
 .publicGalleryTabContainer {
   position: relative;
-  height: 100vh;
+  min-height: 100vh;
 }
 
 .publicGalleryTabOverlayContent {

src/Explorer/Controls/NotebookGallery/GalleryViewerComponent.tsx

@@ -388,7 +388,7 @@ export class GalleryViewerComponent extends React.Component<GalleryViewerCompone
   private createSearchBarHeader(content: JSX.Element): JSX.Element {
     return (
       <Stack tokens={{ childrenGap: 10 }}>
-        <Stack horizontal tokens={{ childrenGap: 20, padding: 10 }}>
+        <Stack horizontal wrap tokens={{ childrenGap: 20, padding: 10 }}>
           <Stack.Item grow>
             <SearchBox value={this.state.searchText} placeholder="Search" onChange={this.onSearchBoxChange} />
           </Stack.Item>

src/Explorer/Controls/NotebookGallery/__snapshots__/GalleryViewerComponent.test.tsx.snap

@@ -36,6 +36,7 @@ exports[`GalleryViewerComponent renders 1`] = `
                 "padding": 10,
               }
             }
+            wrap={true}
           >
             <StackItem
               grow={true}
@@ -121,6 +122,7 @@ exports[`GalleryViewerComponent renders 1`] = `
               "padding": 10,
             }
           }
+          wrap={true}
         >
           <StackItem
             grow={true}