[!!!][TASK] Add configuration error note for single view plugins

[benjaminkott]

To prevent the usage of plugins that should only be
used on post views we are now adding additional
checks for those. If no post could be resolved - also
means if the plugin is used on pages that do not match
the Constants::DOKTYPE_BLOG_POST - the plugins will
now return a new message to make the miss usage visible.

A possible configuration error was detected.
No matching post could be obtained. Make sure that this plugin is only used on a post.

The following plugins will now show this message if no
post could be obtained:

• Authors
• Footer
• Header
• Metadata
• RelatedPosts

Templates added:

• Layouts/Post.html

Templates changed:

• Templates/Comment/Comments.html
• Templates/Comment/Form.html
• Templates/Post/Authors.html
• Templates/Post/Footer.html
• Templates/Post/Header.html
• Templates/Post/Metadata.html
• Templates/Post/RelatedPosts.html

Releases: master