MM-11868/MM-12010/MM-12011/MM-12036 Improve post metadata structure (#9693)

* MM-11868/MM-12010/MM-12011/MM-12036 Include dimensions for opengraph and message attachment images in post metadata

* Remove duplicate check from getFirstLinkAndImages

* Add tests for getImagesInMessageAttachments

* Rename PostMetadata.FileInfos to PostMetadata.Files

* Rename Metadata.ImageDimensions to Metadata.Images

model/post_metadata.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+type PostMetadata struct {
+	// An array of the information required to render additional details about the contents of this post.
+	Embeds []*PostEmbed `json:"embeds,omitempty"`
+
+	// An arrayof the  custom emojis used in the post or in reactions to the post.
+	Emojis []*Emoji `json:"emojis,omitempty"`
+
+	// An array of information about the file attachments on the post.
+	Files []*FileInfo `json:"files,omitempty"`
+
+	// A map of image URL to information about  all external images in the post. This includes image embeds,
+	// inline Markdown images, OpenGraph images, and message attachment images, but it does not contain the dimensions
+	// of file attachments which are contained in PostMetadata.FileInfos.
+	Images map[string]*PostImage `json:"images,omitempty"`
+
+	// A map of emoji names to a count of users that reacted with the given emoji.
+	ReactionCounts ReactionCounts `json:"reaction_counts,omitempty"`
+}
+
+type PostImage struct {
+	Width  int `json:"width"`
+	Height int `json:"height"`
+}