Vue Migration - fixes #80

[berdosi]

This is quite a mouthful, for a full rewrite of the frontend template (API calls were kept pretty much as-is)

• Uses @nextcloud/vue components, giving an uniform feel
• Responsive behavior is reworked (there is no playing around with font sizes, just columns being hidden / shown)
• CSS uses classes instead of IDs.
• Home city is set from the city list.

It builds with npm install && npm build. The bundle is around 1MB, which seems large - but comparable to all the other Nextcloud applications.

Also, I've added a blur to the content panels, which could work around #90 (only works on WebKit):