fix(ui): Add pasted images to dropzone

This adds pasted images to the dropzone to provide the same experience
as when using the dropzone. This gives the possibility to preview and
delete the image. Additionally it provides a copy button to copy the
markdown code for inserting the image.

Removed the old implementation in `repo-legacy.js` and generalized
everything in `common-global.js` as common implementation.

Replaced old jquery code with plain JS.

Fixes #4588

modules/structs/attachment.go

@@ -22,6 +22,12 @@ type Attachment struct {
 	Type string `json:"type"`
 }
 
+// WebAttachment the generic attachment with mime type
+type WebAttachment struct {
+	*Attachment
+	MimeType string `json:"mime_type"`
+}
+
 // EditAttachmentOptions options for editing attachments
 // swagger:model
 type EditAttachmentOptions struct {