Add created_by filter to SearchIssues

This patch adds the created_by filter to the SearchIssues method.

tea cli has an option to filter by author when listing issues, but it's
not working. The tea command line creates this request for the API when
using the author filter:

$ tea issue list -l local --kind pull -A danigm -vvv
http://localhost:3000/api/v1/repos/issues/search?created_by=danigm&labels=&limit=30&milestones=&page=1&state=open&type=pulls

This patch fixes the API to allow this kind of queries from go-sdk and
tea cli.

Author: danigm

routers/api/v1/repo/issue.go

@@ -157,6 +157,10 @@ func SearchIssues(ctx *context.APIContext) {
 	//   in: query
 	//   description: Filter by repository owner
 	//   type: string
+	// - name: created_by
+	//   in: query
+	//   description: Only show items which were created by the given user
+	//   type: string
 	// - name: team
 	//   in: query
 	//   description: Filter by team (requires organization owner parameter)
@@ -257,6 +261,14 @@ func SearchIssues(ctx *context.APIContext) {
 		searchOpt.UpdatedBeforeUnix = optional.Some(before)
 	}
 
+	createdByID := getUserIDForFilter(ctx, "created_by")
+	if ctx.Written() {
+		return
+	}
+	if createdByID > 0 {
+		searchOpt.PosterID = strconv.FormatInt(createdByID, 10)
+	}
+
 	if ctx.IsSigned {
 		ctxUserID := ctx.Doer.ID
 		if ctx.FormBool("created") {