removed katex script tag from main.html, use import instead

browser/lib/markdown.js

@@ -4,10 +4,9 @@ import emoji from 'markdown-it-emoji'
 import math from '@rokt33r/markdown-it-math'
 import _ from 'lodash'
 import ConfigManager from 'browser/main/lib/ConfigManager'
+import katex from 'katex'
 import {lastFindInArray} from './utils'
 
-// FIXME We should not depend on global variable.
-const katex = window.katex
 const config = ConfigManager.get()
 
 function createGutter (str, firstLineNumber) {