Attach to release (#673)

* Moved attachaments POST url from /issues/attachments to /attachments

* Implemented attachment upload on release page

* Implemented downloading attachments on the release page

* Added zip and gzip files to default allowed attachments

* Implemented uploading attachments on edit release

* Renamed UploadIssueAttachment to UploadAttachment

modules/auth/repo_form.go

@@ -267,6 +267,7 @@ type NewReleaseForm struct {
 	Content    string
 	Draft      string
 	Prerelease bool
+	Files   []string
 }
 
 // Validate valideates the fields
@@ -280,6 +281,7 @@ type EditReleaseForm struct {
 	Content    string `form:"content"`
 	Draft      string `form:"draft"`
 	Prerelease bool   `form:"prerelease"`
+	Files   []string
 }
 
 // Validate valideates the fields