Resolve "Support the OpenGraph protocol"

Open Graph Protocol metadata is added to each layout describing the page at hand for use in social media. A variable "layout" is passed via top html file into open_graph_protocol html file. According to the content of the variable different open graph metadata is added to the page.

There are 6 concerns:

1. Is there a nicer way to get the full names of blog post authors by ID?
2. Is the title and description of each page meaningful enough?
3. Is the self-made HIFIS logo acceptable as image representing the project?
4. Is there any other open graph metadata which should be added as well?
5. How to test the results rendered in different social media of each page layout according to the open graph metadata given?
6. Is there anything else I have forgotten?

Closes #48 (closed)