Replace vue-router by a (hopefully) lighter solution
Summary: The idea was to remove the complexity and weight of vue-router and replace it with a lighter tailored solution that makes the code simpler.
Test Plan: I added a set of unit tests that pass with: npm test.
Reviewers: fdans, Milimetric
Reviewed By: fdans
Subscribers: CHUANRU, jenkins
Maniphest Tasks: T170459
Differential Revision: https://phabricator.wikimedia.org/D742