Import Slack bot_message type posts. (#4261)

This includes all messages from integrations, as far as I can tell.

Messages are "owned" by a special user that is deactivated once the
import completes.

Override User Names are only shown where the individual Slack posts have
a username override in them. Ones set centrally through the Slack Web
Hooks administration, or by Slack-official integrations, aren't known so
we can't set them on the imported posts. Same for icons.

The attachment "colors" aren't imported as Mattermost does not appear to
have any equivalent feature.

Fixes PLT-48

api/import.go

@@ -6,6 +6,7 @@ package api
 import (
 	"bytes"
 	"io"
+	"regexp"
 
 	l4g "github.com/alecthomas/log4go"
 	"github.com/mattermost/platform/model"
@@ -80,3 +81,57 @@ func ImportFile(file io.Reader, teamId string, channelId string, userId string,
 
 	return fileInfo, nil
 }
+
+func ImportIncomingWebhookPost(post *model.Post, props model.StringInterface) {
+	linkWithTextRegex := regexp.MustCompile(`<([^<\|]+)\|([^>]+)>`)
+	post.Message = linkWithTextRegex.ReplaceAllString(post.Message, "[${2}](${1})")
+
+	post.AddProp("from_webhook", "true")
+
+	if _, ok := props["override_username"]; !ok {
+		post.AddProp("override_username", model.DEFAULT_WEBHOOK_USERNAME)
+	}
+
+	if len(props) > 0 {
+		for key, val := range props {
+			if key == "attachments" {
+				if list, success := val.([]interface{}); success {
+					// parse attachment links into Markdown format
+					for i, aInt := range list {
+						attachment := aInt.(map[string]interface{})
+						if aText, ok := attachment["text"].(string); ok {
+							aText = linkWithTextRegex.ReplaceAllString(aText, "[${2}](${1})")
+							attachment["text"] = aText
+							list[i] = attachment
+						}
+						if aText, ok := attachment["pretext"].(string); ok {
+							aText = linkWithTextRegex.ReplaceAllString(aText, "[${2}](${1})")
+							attachment["pretext"] = aText
+							list[i] = attachment
+						}
+						if fVal, ok := attachment["fields"]; ok {
+							if fields, ok := fVal.([]interface{}); ok {
+								// parse attachment field links into Markdown format
+								for j, fInt := range fields {
+									field := fInt.(map[string]interface{})
+									if fValue, ok := field["value"].(string); ok {
+										fValue = linkWithTextRegex.ReplaceAllString(fValue, "[${2}](${1})")
+										field["value"] = fValue
+										fields[j] = field
+									}
+								}
+								attachment["fields"] = fields
+								list[i] = attachment
+							}
+						}
+					}
+					post.AddProp(key, list)
+				}
+			} else if key != "from_webhook" {
+				post.AddProp(key, val)
+			}
+		}
+	}
+
+	ImportPost(post)
+}