Add ability to sort issues by due date (#6206) (#6244)

Signed-off-by: Steve Traugott <stevegt@t7a.org>

models/issue.go

 		sess.Asc("issue.num_comments")
 	case "priority":
 		sess.Desc("issue.priority")
+	case "nearduedate":
+		sess.Asc("issue.deadline_unix")
+	case "farduedate":
+		sess.Desc("issue.deadline_unix")
 	default:
 		sess.Desc("issue.created_unix")
 	}

options/locale/locale_en-US.ini

 issues.filter_sort.leastupdate = Least recently updated
 issues.filter_sort.mostcomment = Most commented
 issues.filter_sort.leastcomment = Least commented
+issues.filter_sort.nearduedate = Nearest due date
+issues.filter_sort.farduedate = Farthest due date
 issues.filter_sort.moststars = Most stars
 issues.filter_sort.feweststars = Fewest stars
 issues.filter_sort.mostforks = Most forks

templates/repo/issue/list.tmpl

 							<a class="{{if eq .SortType "leastupdate"}}active{{end}} item" href="{{$.Link}}?q={{$.Keyword}}&type={{$.ViewType}}&sort=leastupdate&state={{$.State}}&labels={{.SelectLabels}}&milestone={{$.MilestoneID}}&assignee={{$.AssigneeID}}">{{.i18n.Tr "repo.issues.filter_sort.leastupdate"}}</a>
 							<a class="{{if eq .SortType "mostcomment"}}active{{end}} item" href="{{$.Link}}?q={{$.Keyword}}&type={{$.ViewType}}&sort=mostcomment&state={{$.State}}&labels={{.SelectLabels}}&milestone={{$.MilestoneID}}&assignee={{$.AssigneeID}}">{{.i18n.Tr "repo.issues.filter_sort.mostcomment"}}</a>
 							<a class="{{if eq .SortType "leastcomment"}}active{{end}} item" href="{{$.Link}}?q={{$.Keyword}}&type={{$.ViewType}}&sort=leastcomment&state={{$.State}}&labels={{.SelectLabels}}&milestone={{$.MilestoneID}}&assignee={{$.AssigneeID}}">{{.i18n.Tr "repo.issues.filter_sort.leastcomment"}}</a>
+							<a class="{{if eq .SortType "nearduedate"}}active{{end}} item" href="{{$.Link}}?q={{$.Keyword}}&type={{$.ViewType}}&sort=nearduedate&state={{$.State}}&labels={{.SelectLabels}}&milestone={{$.MilestoneID}}&assignee={{$.AssigneeID}}">{{.i18n.Tr "repo.issues.filter_sort.nearduedate"}}</a>
+							<a class="{{if eq .SortType "farduedate"}}active{{end}} item" href="{{$.Link}}?q={{$.Keyword}}&type={{$.ViewType}}&sort=farduedate&state={{$.State}}&labels={{.SelectLabels}}&milestone={{$.MilestoneID}}&assignee={{$.AssigneeID}}">{{.i18n.Tr "repo.issues.filter_sort.farduedate"}}</a>
 						</div>
 					</div>
 				</div>

templates/user/dashboard/issues.tmpl

 							<a class="{{if eq .SortType "leastupdate"}}active{{end}} item" href="{{$.Link}}?type={{$.ViewType}}&repo={{.RepoID}}&sort=leastupdate&state={{$.State}}">{{.i18n.Tr "repo.issues.filter_sort.leastupdate"}}</a>
 							<a class="{{if eq .SortType "mostcomment"}}active{{end}} item" href="{{$.Link}}?type={{$.ViewType}}&repo={{.RepoID}}&sort=mostcomment&state={{$.State}}">{{.i18n.Tr "repo.issues.filter_sort.mostcomment"}}</a>
 							<a class="{{if eq .SortType "leastcomment"}}active{{end}} item" href="{{$.Link}}?type={{$.ViewType}}&repo={{.RepoID}}&sort=leastcomment&state={{$.State}}">{{.i18n.Tr "repo.issues.filter_sort.leastcomment"}}</a>
+							<a class="{{if eq .SortType "nearduedate"}}active{{end}} item" href="{{$.Link}}?type={{$.ViewType}}&repo={{.RepoID}}&sort=nearduedate&state={{$.State}}">{{.i18n.Tr "repo.issues.filter_sort.nearduedate"}}</a>
+							<a class="{{if eq .SortType "farduedate"}}active{{end}} item" href="{{$.Link}}?type={{$.ViewType}}&repo={{.RepoID}}&sort=farduedate&state={{$.State}}">{{.i18n.Tr "repo.issues.filter_sort.farduedate"}}</a>
 						</div>
 					</div>
 				</div>