Add announcement banner

[svedova]

Create an announcement banner which shows on top of every page.

Proposal

The idea behind this is being able to easily communicate with visitors an important message. To make things more dynamic, I would also propose to rely on the remote configuration. So we can display the content only if there is a remote configuration (something like appBanner) enabled.

Implementation Plan

• [ ] Create an AppBanner.vue component
• [ ] Import the component in pages/index.vue and display it only if the remote config is defined.