Codex components part 1

This MR does the following:

• Adds LESS and, @wikimedia/codex-design-tokens as dependencies to the project
• Adds some global CSS styling (mostly resets)
• Moves the navigation element into a new stand-alone component, simplifying the App.vue file
• Renames the top-level router view components to include "page", to help differentiate them from the smaller components used inside of them. LoginPage, OverviewPage, etc.
• Re-works the JobHistory, Backport, and Navigation components to use the Vue Composition API (this allowed for some simpler code, especially in the templates)
• Replaces the custom table in JobHistory with a Codex Table component; an inline Message component is also used to display job status. Instead of emitting events on row clicks, the job IDs are now <router-link> components that link to the job detail page for the relevant job.
• Updates the Backport component to use a Codex ChipInput component combined with a Button inside of a Field component for the Gerrit patch lookup feature. In the future this element might be a good place for the new Codex MultiselectLookup component, but we need a way to query an API to get Gerrit patch numbers first. This component has also been re-worked to rely on computed properties for things like automatically enabling/disabling the button.