Title and description tags SEO (7666, 7667)

This makes some changes to `meta` and `title` tags for search engine optimization. It addresses #7666 and #7667.

[#7666](https://issues.dp.la/issues/7666) (Remove "Welcome" from front page title)
Before these changes, the home page contained these tags:
`<meta content="Welcome · Digital Public Library of America" property="og:title" />`
`<title>Welcome · Digital Public Library of America</title>`

After these changes, the home page contains these tags:
`<meta content="Digital Public Library of America" property="og:title" />`
`<title>Digital Public Library of America</title>`

[#7667](https://issues.dp.la/issues/7667) (Add meta description tag)
This introduces the following to the `head` of the homepage:
`<meta content="The Digital Public Library of America brings together the riches of America’s libraries, archives, and museums, and makes them freely available to the world." name="description" />`

The home page already has the following tag: 
`<meta content="The Digital Public Library of America brings together the riches of America’s libraries, archives, and museums, and makes them freely available to the world." property="og:description" />`

However, this tag (above) is not being picked up by Google, possibly because the website was not fully Open Graph compliant before these changes.  In order to be Open Graph compliant, these tags are introduced to every page:
`<meta content="website" property="og:type" />`
` <meta content="[...']" property="og:url" />`