Fix mobile cursor from jumping to bottom

When selecting text on the android, it was possible to also highlight the text inside the buttons in the adder. When this happened, the range would grab everything from where you started to highlight to the very end of the dom because the adder is attached as the last node inside body.

The simple fix is to prohibit text selection inside the adder which prevents the selection from getting the adder itself.

src/styles/annotator/components/adder.scss

@@ -22,6 +22,12 @@ $adder-transition-duration: 80ms;
   background: var.$color-background;
   border-radius: var.$annotator-border-radius;
 
+  // Prevent the browser from selecting text in the adder itself during text
+  // selection.
+  //
+  // See https://github.com/hypothesis/product-backlog/issues/878
+  user-select: none;
+
   // The adder toolbar has a distinctive, broad-spreading drop shadow
   box-shadow: 0px 2px 10px 0px rgba(0, 0, 0, 0.25);