Issue due date api (#3890)

* Implemented basic api endpoint to manage deadlines

* Fixed checking for permissions

* Updating a deadline from the ui is now entirely done via the api

* cleanup

* Cosmetics

* fixed lint + fmt

* Added swagger model definition for deadline response

* Updated gitea-sdk

* Updated gitea-sdk

* More cleanup

* Generate swagger json

* Merge branch 'master' of https://github.com/go-gitea/gitea into issue-due-date-api

# Conflicts:
#	public/swagger.v1.json

* Fixed permission to update a deadline via api

* Re-added form to change a deadline

* Added client-side validation + not ignore error messages from the api

* Added locale for error message

* Merge branch 'master' of https://github.com/go-gitea/gitea

# Conflicts:
#	models/issue_comment.go

* Proper date validation

* Fixed indention

* moved css to css file

* added documentation for error codes

* after merge cleanup

* Added swagger description

* DO NOTHING BUT TRIGGER THAT F*CKIN CI SO IT PICKS UP THE LATEST COMMIT AS IT SHOULD

* DO NOTHING BUT TRIGGER THAT F*CKIN CI SO IT PICKS UP THE LATEST COMMIT AS IT SHOULD

* regenerated stylesheets

options/locale/locale_en-US.ini

@@ -781,6 +781,7 @@ issues.due_date_added = "added the due date %s %s"
 issues.due_date_modified = "modified the due date to %s from %s %s"
 issues.due_date_remove = "removed the due date %s %s"
 issues.due_date_overdue = "Overdue"
+issues.due_date_invalid = "The due date is invalid or out of range. Please use the format yyyy-mm-dd."
 
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