[Security] Bump bootstrap from 3.4.1 to 4.3.1

Bumps [bootstrap](https://github.com/twbs/bootstrap) from 3.4.1 to 4.3.1. **This update includes security fixes.**
- [Release notes](https://github.com/twbs/bootstrap/releases)
- [Commits](https://github.com/twbs/bootstrap/compare/v3.4.1...v4.3.1)

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

package-lock.json

@@ -1415,9 +1415,9 @@
       "dev": true
     },
     "bootstrap": {
-      "version": "3.4.1",
-      "resolved": "http://registry.npmjs.org/bootstrap/-/bootstrap-3.4.1.tgz",
-      "integrity": "sha512-yN5oZVmRCwe5aKwzRj6736nSmKDX7pLYwsXiCj/EYmo16hODaBiT4En5btW/jhBF/seV+XMx3aYwukYC3A49DA=="
+      "version": "4.3.1",
+      "resolved": "http://registry.npmjs.org/bootstrap/-/bootstrap-4.3.1.tgz",
+      "integrity": "sha512-rXqOmH1VilAt2DyPzluTi2blhk17bO7ef+zLLPlWvG494pDxcM234pJ8wTc/6R40UWizAIIMgxjvxZg5kmsbag=="
     },
     "brace-expansion": {
       "version": "1.1.11",

package.json

@@ -27,7 +27,7 @@
     "autosize": "^4.0.2",
     "backbone": "^1.3.3",
     "blueimp-md5": "^2.10.0",
-    "bootstrap": "^3.4.1",
+    "bootstrap": "^4.3.1",
     "clipboard": "^2.0.4",
     "css-vars-ponyfill": "^1.17.0",
     "davclient.js": "git+https://github.com/owncloud/davclient.js.git#0.1.3",