Add metatag to show a relevant image on each page

[gabaldon]

The website should include a different meta tags image for each page. Example:

[Tommytrg]

Currently, we are showing the same image on all routes:

We should include a specific metatag for the following routes:

• https://feeds.witnet.io/:network -> network image
• https://feeds.witnet.io/:network/:pricefeed -> price feed image