unify terms to 'spreads'

This commit only renames things, changes no logic.  Any mention of "book", "view", or "mode" is renamed in relation to "spreads".

The AnchoredBox.jsx file is renamed to Anchored.jsx

Extra icons are deleted, and the remaining ones are renamed.

client/components/Anchored.jsx

@@ -1,5 +1,5 @@
 import React, { useState, useRef, forwardRef, useEffect, cloneElement, Children } from 'react';
-import './anchoredBox.less';
+import './Anchored.less';
 
 // Anchored is a wrapper component that must have as children an <AnchoredTrigger> and a <AnchoredBox> component.
 // AnchoredTrigger must have a unique `id` prop, which is passed up to Anchored, saved in state on mount, and