Add summary and picture for tags

[fogrew]

DoD

• tags pages have image and description in meta-tags (don't remind about open graph Add meta tags for sharing #42 )
• if a article page does not have its own picture, the picture of the main tag should be displayed in a articles lists

// the main tag is the one in which the articles are listed
// for example, on the code page it is a projects for first list and snippets for second list

Pull request
see #44