Revert "Adjust webpack config to new code mirror version"

It turned out that we can't directly use the codemirror source files and actually need to run their build script manually.

This reverts commit 0ec180de

Signed-off-by: David Mehren <git@herrmehren.de>

package.json

@@ -29,7 +29,7 @@
     "bootstrap-validator": "^0.11.8",
     "chance": "^1.0.4",
     "cheerio": "^0.22.0",
-    "codemirror": "git+https://github.com/codimd/CodeMirror.git",
+    "codemirror": "git+https://github.com/hackmdio/CodeMirror.git",
     "compression": "^1.6.2",
     "connect-flash": "^0.1.1",
     "connect-session-sequelize": "^6.0.0",