Fix npm audit reports

[sequba]

Description

npm audit reports 2 vulnerabilities, some of which are in the dependencies of vuepress. To fix them, vuepress needs to be updated to version 2 and our code adjusted because of major breaking changes: https://v2.vuepress.vuejs.org/guide/migration.html

But Vuepress v2 is still in the beta stage, so in my opinion it is better to wait for a stable release.

Links

#1239
#1262