Disable merging a WIP Pull request (#4529)

* prevent pull request to be merged when PR is a WIP

* add tests

* add helper to prepend WIP: in PR title

* move default wip prefixes into settings

* use configurable WIP prefixes in javascript and default to first one in templates

* add documentation

* add unit test on pull model

Signed-off-by: Julien Tant <julien@craftyx.fr>

routers/repo/issue.go

@@ -356,6 +356,7 @@ func NewIssue(ctx *context.Context) {
 	ctx.Data["RequireHighlightJS"] = true
 	ctx.Data["RequireSimpleMDE"] = true
 	ctx.Data["RequireTribute"] = true
+	ctx.Data["PullRequestWorkInProgressPrefixes"] = setting.Repository.PullRequest.WorkInProgressPrefixes
 	setTemplateIfExists(ctx, issueTemplateKey, IssueTemplateCandidates)
 	renderAttachmentSettings(ctx)
 
@@ -449,6 +450,7 @@ func NewIssuePost(ctx *context.Context, form auth.CreateIssueForm) {
 	ctx.Data["RequireHighlightJS"] = true
 	ctx.Data["RequireSimpleMDE"] = true
 	ctx.Data["ReadOnly"] = false
+	ctx.Data["PullRequestWorkInProgressPrefixes"] = setting.Repository.PullRequest.WorkInProgressPrefixes
 	renderAttachmentSettings(ctx)
 
 	var (