GH#1737 Retain '@' when replacing usernames (#1985)

* GH#1737 Retain '@' when replacing usernames

See issue #1737 for context.

Currently it's not possible from the skill code to distinguish between
`@botname message` and `botname message`. With this change, username mentions
retain the `@` character.

* Fix test

---------

Co-authored-by: Jacob Tomlinson <jacob@tomlinson.email>

opsdroid/connector/slack/connector.py

@@ -444,7 +444,8 @@ async def replace_usernames(self, message):
         for userid in userids:
             user_info = await self.lookup_username(userid)
             message = message.replace(
-                "<@{userid}>".format(userid=userid), user_info["name"]
+                "<@{userid}>".format(userid=userid),
+                "@{username}".format(username=user_info["name"]),
             )
 
         return message

opsdroid/connector/slack/tests/test_connector.py

@@ -295,7 +295,7 @@ async def test_replace_usernames(connector):
     connector.known_users = {"U01NK1K9L68": {"name": "Test User"}}
     message = "hello <@U01NK1K9L68>"
     replaced_message = await connector.replace_usernames(message)
-    assert replaced_message == "hello Test User"
+    assert replaced_message == "hello @Test User"
 
 
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