feat: Switch to GitLab's vue-toasted and enable keepOnHover by default

What does this MR do?

It replaces vue-toasted's official version with our own fork that supports a keepOnHover option to prevent toasts from closing while the mouse is over them.

Why is it needed?

vue-toasted currently does not support the hover functionality we need. We opened a PR to implement it but the original project is seemingly inactive (last commit to master was on January 29th`).

After some discussion, we have decided to temporarily use our own fork of vue-toasted.

We will then start working on a switch to BootstrapVue's toast component. Since BootstrapVue's component is currently in beta, we'll wait until it stabilizes before considering to ship it as part of gitlab-ui.

Notes

Some commits from the original projects were never actually released and ended being bundled into our own package. One of the unreleased commits added some padding on the toasts, which broke our snapshots tests. To prevent any regression, this commit was reverted in our fork.

Closes #238 (closed)