Improve alert management client-side router usage

Alert management uses vue-router for tab navigation.

It is currently set up to use hash mode - this results in a URL that looks like: <base>/root/test-proj/-/alert_management/2/details#/overview

A nicer user experience would be to not have a hash, so the URL looks like: <base>/root/test-proj/-/alert_management/2/details/overview

It would also be nice to have the forward/back buttons working.

Difficulties

This would require both:

1. a frontend change to update the router with the new settings.
2. a backend change, so that the supported routes forward to the correct base page.

Proposal:

• Use history mode instead of hash mode for the alert management client-side router.
• Use push instead of replace to enable the forward/back buttons.

Edge cases

Ensure that non-default base paths work (e.g https://www.some-host.com/gitlab)