reposition author/date location in _posts updates

[nicsrlv]

re-position author/date field to justify at top of _post pages, instead of middle.

[joncamfield]

• Blog post listing page does not show posted dates
• Blog post pages themselves center-align author/date in the middle of a potentially very long post (see https://dev.level-up.cc/news/2016-07-11-new-levelup-site-launch/)
• Blog post pages repeat the preview?

[joncamfield]

Center align is caused by a flexbox rule on

to center align; removing that and adding
align-items: flex-start;

should top-align both the post meta data (author, date) with the post title)