Issue due date (#3794)

* Started adding deadline to ui

* Implemented basic issue due date managing

* Improved UI for due date managing

* Added at least write access to the repo in order to modify issue due dates

* Ui improvements

* Added issue comments creation when adding/modifying/removing a due date

* Show due date in issue list

* Added api support for issue due dates

* Fixed lint suggestions

* Added deadline to sdk

* Updated css

* Added support for adding/modifiying deadlines for pull requests via api

* Fixed comments not created when updating or removing a deadline

* update sdk (will do properly once go-gitea/go-sdk#103 is merged)

* enhanced updateIssueDeadline

* Removed unnessecary Issue.DeadlineString

* UI improvements

* Small improvments to comment creation
+ ui & validation improvements

* Check if an issue is overdue is now a seperate function

* Updated go-sdk with govendor as it was merged

* Simplified isOverdue method

* removed unessecary deadline to 0 set

* Update swagger definitions

* Added missing return

* Added an explanary comment

* Improved updateIssueDeadline method so it'll only update `deadline_unix`

* Small changes and improvements
* no need to explicitly load the issue when updating a deadline, just use whats already there
* small optimisations

* Added check if a deadline was modified before updating it

* Moved comment creating logic into its own function

* Code cleanup for creating deadline comment

* locale improvement

* When modifying a deadline, the old deadline is saved with the comment

* small improvments to xorm session handling when updating an issue deadline
+ style nitpicks

* style nitpicks

* Moved checking for if the user has write acces to middleware

models/issue_comment.go

@@ -60,6 +60,12 @@ const (
 	CommentTypeAddTimeManual
 	// Cancel a stopwatch for time tracking
 	CommentTypeCancelTracking
+	// Added a due date
+	CommentTypeAddedDeadline
+	// Modified the due date
+	CommentTypeModifiedDeadline
+	// Removed a due date
+	CommentTypeRemovedDeadline
 )
 
 // CommentTag defines comment tag type
@@ -485,6 +491,34 @@ func createAssigneeComment(e *xorm.Session, doer *User, repo *Repository, issue
 	})
 }
 
+func createDeadlineComment(e *xorm.Session, doer *User, issue *Issue, newDeadlineUnix util.TimeStamp) (*Comment, error) {
+
+	var content string
+	var commentType CommentType
+
+	// newDeadline = 0 means deleting
+	if newDeadlineUnix == 0 {
+		commentType = CommentTypeRemovedDeadline
+		content = issue.DeadlineUnix.Format("2006-01-02")
+	} else if issue.DeadlineUnix == 0 {
+		// Check if the new date was added or modified
+		// If the actual deadline is 0 => deadline added
+		commentType = CommentTypeAddedDeadline
+		content = newDeadlineUnix.Format("2006-01-02")
+	} else { // Otherwise modified
+		commentType = CommentTypeModifiedDeadline
+		content = newDeadlineUnix.Format("2006-01-02") + "|" + issue.DeadlineUnix.Format("2006-01-02")
+	}
+
+	return createComment(e, &CreateCommentOptions{
+		Type:    commentType,
+		Doer:    doer,
+		Repo:    issue.Repo,
+		Issue:   issue,
+		Content: content,
+	})
+}
+
 func createChangeTitleComment(e *xorm.Session, doer *User, repo *Repository, issue *Issue, oldTitle, newTitle string) (*Comment, error) {
 	return createComment(e, &CreateCommentOptions{
 		Type:     CommentTypeChangeTitle,