Use Material icons for general and network settings

Signed-off-by: Felix Weilbach <felix.weilbach@nextcloud.com>

theme/network.svg

@@ -1,10 +1 @@
-<?xml version="1.0" encoding="UTF-8" standalone="no"?>
-<svg xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://www.w3.org/2000/svg" height="64" width="64" version="1.0" xmlns:cc="http://creativecommons.org/ns#" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:dc="http://purl.org/dc/elements/1.1/">
- <defs>
-  <linearGradient id="a" y2="62" gradientUnits="userSpaceOnUse" x2="32" gradientTransform="translate(0,1)" x1="32">
-   <stop offset="0"/>
-   <stop stop-color="#363636" offset="1"/>
-  </linearGradient>
- </defs>
- <path opacity=".7" d="m20 1-20 20h12v14h16v-14h12zm16 28v14h-12l20 20 20-20h-12v-14z" fill="url(#a)"/>
-</svg>
+<svg xmlns="http://www.w3.org/2000/svg" height="24px" viewBox="0 0 24 24" width="24px" fill="#000000"><path d="M0 0h24v24H0z" fill="none"/><path d="M16 17.01V10h-2v7.01h-3L15 21l4-3.99h-3zM9 3L5 6.99h3V14h2V6.99h3L9 3z"/></svg>