Make sure IsPost gets set correctly for tags and archive pages

[daveaglick]

Those pages are not themselves posts, even though the IsPost metadata might flow up and get set for them. Once IsPost is correctly cleared for these types of pages we can remove the title checking at https://github.com/statiqdev/CleanBlog/commit/76608e64427ff4d89344c224ffb646009d8c99f2#diff-cfbc7d2953501a88d12bed04ae71f6d94c35bdef434ea5c32b0ec37d5568fbcaR9 in the CleanBlog theme.

[aycippo]

apologies for asking a question that may have been previously answered, but what part of the project is in charge of handing the work mentioned in this response? thanks! https://github.com/statiqdev/CleanBlog/commit/76608e64427ff4d89344c224ffb646009d8c99f2#commitcomment-81529328