[mastodon] improve account searches (fixes #704)

Searching for just the username ("@name") can produce multiple
unrelated results, so we now search for username + mastodon instance
("@name@INSTANCE")

gallery_dl/extractor/mastodon.py

@@ -68,8 +68,8 @@ def __init__(self, match):
         self.account_name = match.group(1)
 
     def statuses(self):
-        results = self.api.account_search("@" + self.account_name, 1)
-        for account in results:
+        handle = "@{}@{}".format(self.account_name, self.instance)
+        for account in self.api.account_search(handle, 1):
             if account["username"] == self.account_name:
                 break
         else: