Change loading icon for epub

[mbakeryo]

The current (honeycomb) icon creates problems for people with trypophobia.

Find a new icon that doesn't have holes or patterns.

[Jonathan McGlone]

Here's a start at a Loading icon that uses the Fulcrum logo:

https://codepen.io/jwmcglone/pen/BxyMqB

[Jonathan McGlone]

Resolved by PR #1825