Add Announcements tab, powered by notify.js

[oleg-nenashev]

Adds support for showing the announcements in the top of then site. They will be shown once within the JavaScript context (window or so)

Submitter checklist

• The PR request is well described and justified, including the body and the references
• The PR title represents the desired changelog entry
• If the change against WireMock 2 functionality (incompatible with WireMock 3),
  it is submitted against the 2.x branch
• The repository's code style is followed (see the contributing guide)

Details: Contributor Guide