Fix !channel linking in Slack import. (#4065)

Original version of the patch didn't take into account that, like with
@mentions, there's an "old" and a "new" format in the Slack export files
for channel mentions. This version imports both correctly.

i18n/en.json

@@ -1511,6 +1511,10 @@
     "id": "api.slackimport.slack_convert_user_mentions.compile_regexp_failed.warn",
     "translation": "Failed to compile the @mention matching regular expression for Slack user {{.UserID}} {{.Username}}"
   },
+  {
+    "id": "api.slackimport.slack_convert_channel_mentions.compile_regexp_failed.warn",
+    "translation": "Failed to compile the !channel matching regular expression for Slack channel {{.ChannelID}} {{.ChannelName}}"
+  },
   {
     "id": "api.slackimport.slack_import.log",
     "translation": "Mattermost Slack Import Log\r\n"