Bump lodash from 4.17.4 to 4.17.21

Bumps [lodash](https://github.com/lodash/lodash) from 4.17.4 to 4.17.21.
- [Release notes](https://github.com/lodash/lodash/releases)
- [Commits](lodash/lodash@4.17.4...4.17.21)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package-lock.json

@@ -22,7 +22,7 @@
     "gravatar": "^1.0.6",
     "jade": "~1.11.0",
     "jsonwebtoken": "^7.3.0",
-    "lodash": "^4.17.4",
+    "lodash": "^4.17.21",
     "marked": "^0.3.3",
     "mkdirp": "^0.5.0",
     "moment": "^2.18.0",