refactor: image handling logic + support for external images (#1235)

* refactor: simplify image partial logic, remove generation of different image sizes

* Remove `defaultImage.opengraph` configuration from `params.toml`.

layouts/_partials/head/opengraph/provider/twitter.html

@@ -9,8 +9,8 @@
 <meta name="twitter:title" {{ printf "content=%q" $title | safeHTMLAttr }}>
 <meta name="twitter:description" {{ printf "content=%q" $description | safeHTMLAttr }}>
 
-{{- $image := partialCached "helper/image" (dict "Context" . "Type" "opengraph") .RelPermalink "opengraph" -}}
-{{- if $image.exists -}}
+{{- $image := partial "helper/image" (dict "Image" .Params.image "Resources" .Resources) -}}
+{{- if $image -}}
     <meta name="twitter:card" content="{{ default `summary_large_image` .Site.Params.opengraph.twitter.card }}">
     <meta name="twitter:image" content='{{ absURL $image.permalink }}' />
 {{- end -}}