Add themes example

[cristijora]

This is a follow up PR after a discussion with @yyx990803 for a partnership with Creative Tim.

We added a Themes page under Examples with a specific query param for each link allowing to track potential sales coming from Vue.js website.

Vue.js will get 50% of each sale and therefore increase it's Open Collective or Patreon revenue and further support Open Source development.

PR breakdown:

• Add new page under "Examples" with Themes
• Add new feature that will allow displaying new/updated links by having a red dot near a link
  
  The feature is built in such a way that any link from the navbar can be added

links = [
      {
        title: 'Learn',
        updatedOn: new Date("Fri Mar 1 2019")
      },
      {
        title: 'Examples',
        updatedOn: new Date("Fri Mar 1 2019")
      }
    ]

The red dot will appear for 2 months starting from updatedOn date.
The red dot near the link will disappear as soon as you visit the page and will no longer appear if you visited the corresponding page. This can be used on any links to show that something was updated.
To add a new link go to common.js -> initNewNavLinks and add the link to the array above.
E.g

 {
        title: 'Ecosystem',
        updatedOn: new Date("Sat Mar 2 2019")
 }

[yyx990803]

Should not remove the current class here

[yyx990803]

Somehow the scrolling-to-header in the sidebar links isn't working for these h3 titles

[cristijora]

Fixed header scrolling and with this occasion updated to smooth-scroll v15.2.1
which supports callback to specify different offsets depending on each link