Add API management for issue/pull and comment attachments (#21783)

Close #14601
Fix #3690

Revive of #14601.
Updated to current code, cleanup and added more read/write checks.

Signed-off-by: Andrew Thornton <art27@cantab.net>
Signed-off-by: Andre Bruch <ab@andrebruch.com>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Norwin <git@nroo.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/convert/attachment.go

@@ -0,0 +1,30 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package convert
+
+import (
+	repo_model "code.gitea.io/gitea/models/repo"
+	api "code.gitea.io/gitea/modules/structs"
+)
+
+// ToAttachment converts models.Attachment to api.Attachment
+func ToAttachment(a *repo_model.Attachment) *api.Attachment {
+	return &api.Attachment{
+		ID:            a.ID,
+		Name:          a.Name,
+		Created:       a.CreatedUnix.AsTime(),
+		DownloadCount: a.DownloadCount,
+		Size:          a.Size,
+		UUID:          a.UUID,
+		DownloadURL:   a.DownloadURL(),
+	}
+}
+
+func ToAttachments(attachments []*repo_model.Attachment) []*api.Attachment {
+	converted := make([]*api.Attachment, 0, len(attachments))
+	for _, attachment := range attachments {
+		converted = append(converted, ToAttachment(attachment))
+	}
+	return converted
+}