fix: package.json & package-lock.json to reduce vulnerabilities (#873)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-AJV-584908
- https://snyk.io/vuln/SNYK-JS-JQUERY-565129
- https://snyk.io/vuln/SNYK-JS-JQUERY-567880
- https://snyk.io/vuln/SNYK-JS-SELECT2-456562
- https://snyk.io/vuln/SNYK-JS-SOCKJS-575261
- https://snyk.io/vuln/SNYK-JS-WEBPACKDEVSERVER-72405
- https://snyk.io/vuln/SNYK-JS-YARGSPARSER-560381
- https://snyk.io/vuln/npm:mem:20180117

package.json

@@ -24,10 +24,10 @@
     "flag-icon-css": "^2.9.0",
     "geolib": "^2.0.24",
     "icheck": "~1.0",
-    "jquery": "~3.4",
+    "jquery": "~3.5.0",
     "jquery-pjax": "~2.0",
     "js-yaml": "^3.13",
-    "laravel-mix": "^2.1.14",
+    "laravel-mix": "^5.0.5",
     "leaflet": "^1.3.4",
     "leaflet-ajax": "2.1.0",
     "leaflet-providers": "1.0.*",
@@ -43,10 +43,10 @@
     "pjax": "^0.2.8",
     "popper.js": "^1.15.0",
     "rivets": "^0.9.6",
-    "select2": "^4.0.5",
+    "select2": "^4.0.8",
     "ssri": "^5.3.0",
     "tar": ">=4.4.2",
-    "webpack": "^3.12.0",
+    "webpack": "^4.0.0",
     "webpack-cli": "^3.3.12",
     "x-editable": "1.5.1"
   },