Date of post

Post have not only title and category but also date.
PostIndexTemplate defined with list comprehension

To do: as above, plus
* deal with 'href' using url_for() in Flask
Blog index created without using all_posts

all_posts is created in the class and not in the website file
As above - plus typo
Use url_for to give href when creating instance of page

To do: use it class, do not repeat.