SEO (7666, 7667, 7669)

This makes some changes to `meta`, `title`, and header (`h1`) tags for search engine optimization. It addresses #7666, #7667, and #7669.

[#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" />`

[#7669](https://issues.dp.la/issues/7669) (More descriptive search results header and title)
Before these changes, the headers (`h1` tags) for the search results pages were "Search Results", "Timeline", "Map", or "Bookshelf", depending on the display interface.  The `title` tags consisted of the query term for the search.  After these changes, the `h1` and `title` tags contain the query and filter terms for the search.

This code also excludes query terms from the message on the search results pages that tells the user how many results their search returned.  The query terms are excluded to avoid redundancy with the new page headers.

Note:  Since one of the edited files is and initializer file, so you will need to restart Rails in order for this file to configure.