Unify news posts and blog posts, and standardise "authors" vs "author" in frontmatter (#695)

Closes #128; this PR adds author and date icons to news posts.

There has been some inconsistency in how authors are mentioned in the
YAML frontmatter, which we have standardised with this PR.

#### use of `authors: ["foo"]`

https://github.com/search?q=%22authors%3A+%5B%22+%28org%3Ascientific-python+OR+repo%3Anumpy%2Fnumpy.org+OR+repo%3Ascipy%2Fscipy.org%29+language%3AMarkdown&type=code

#### use of `author: ["foo"]`

https://github.com/search?q=%22author%3A+%5B%22+%28org%3Ascientific-python+OR+repo%3Anumpy%2Fnumpy.org+OR+repo%3Ascipy%2Fscipy.org%29+language%3AMarkdown&type=code

This PR makes it `authors:` only, which can imply either a single author
or multiple authors.

Author: agriyakhetarpal

layouts/news/list.html

@@ -0,0 +1,3 @@
+{{ define "main" }}
+{{ partial "posts/list.html" . }}
+{{ end }}

layouts/news/single.html

@@ -0,0 +1,3 @@
+{{ define "main" }}
+{{ partial "posts/post.html" . }}
+{{ end }}

layouts/partials/posts/meta.html

@@ -1,6 +1,9 @@
+{{- if .Params.Author -}}
+  {{- errorf "Page %q uses deprecated 'author' field. Please use 'authors' instead." .File.Path -}}
+{{- end -}}
 <div class="post-meta">
 <span class="post-authors">
-    {{- range .Params.author -}}
+    {{- range .Params.Authors -}}
     {{ $author := trim (index (findRE `[^<]*` . 1) 0) "\n\r " }}
     {{ $handle := index (index (findRESubmatch `<(.*?)>` . 1) 0) 1 }}
     <div class="post-author">

layouts/partials/section/section.html

@@ -9,6 +9,9 @@ <h5 class="subtitle">{{ .Params.Subtitle }}</h5>
   {{ .Content }}
   {{ else }}
   {{ range .Pages }}
+  {{- if .Params.Author -}}
+    {{- errorf "Page %q uses deprecated 'author' field. Please use 'authors' instead." .File.Path -}}
+  {{- end -}}
   <div class="post-list">
     <article>
       <div class="post-title"><a href="{{ .RelPermalink }}">{{ .Title }}</a></div>