Make "Add sketch" copy consistent

Buttons and menu items should say "Add sketch" (singular) and titles
should say "Add to collection" as it's implied by context that it's
a sketch.

client/modules/IDE/components/CollectionList/CollectionList.jsx

@@ -142,7 +142,7 @@ class CollectionList extends React.Component {
         {
           this.state.addingSketchesToCollectionId && (
             <Overlay
-              title="Add sketches"
+              title="Add sketch"
               actions={<SketchSearchbar />}
               closeOverlay={this.hideAddSketches}
               isFixedHeight

client/modules/IDE/components/CollectionList/CollectionListRow.jsx

@@ -135,7 +135,7 @@ class CollectionListRowBase extends React.Component {
                 onBlur={this.onBlurComponent}
                 onFocus={this.onFocusComponent}
               >
-                Add sketches
+                Add sketch
               </button>
             </li>
             {userIsOwner &&

client/modules/IDE/pages/IDEView.jsx

@@ -380,7 +380,7 @@ class IDEView extends React.Component {
         {this.props.location.pathname.match(/add-to-collection$/) &&
           <Overlay
             ariaLabel="add to collection"
-            title="Add sketch to collection"
+            title="Add to collection"
             previousPath={this.props.ide.previousPath}
             actions={<CollectionSearchbar />}
             isFixedHeight

client/modules/User/components/Collection.jsx

@@ -325,7 +325,7 @@ class Collection extends React.Component {
           {
             this.state.isAddingSketches && (
               <Overlay
-                title="Add sketches"
+                title="Add sketch"
                 actions={<SketchSearchbar />}
                 closeOverlay={this.hideAddSketches}
                 isFixedHeight