Restyle navigation bar icons

[mharrigan26]

Overview

Restyle the open/close icons in the side navigation bar. The side menu is opened with the menu icon and pushed shut with the same icon overlaid with an arrow. This is more intuitive iconography for the expand/collapse function of the nav bar. New icons generated on https://icomoon.io/app/#/select:

Testing recommendations

Make sure icons appear in nav bar

GitHub issue number

#1257

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;
• Update rebar.config.script with the correct tag once a new Fauxton release is made

[Antonio-Maranhao]

Alternative since the nav bar expands as opposed to opening on top of the main content.