Unfurl Focalboard Server PR (#18271)

* Allow Embeds editing without editing

* Add focalboard to embed after it being in props

* Fix tests

* change to boards

* remove extra gunk

* Fix tests

* Add Feature Flag

* update boardsunfurl

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/post_embed.go

@@ -9,6 +9,7 @@ const (
 	PostEmbedOpengraph         PostEmbedType = "opengraph"
 	PostEmbedLink              PostEmbedType = "link"
 	PostEmbedPermalink         PostEmbedType = "permalink"
+	PostEmbedBoards            PostEmbedType = "boards"
 )
 
 type PostEmbedType string