Fix check on conversation size

Before, we were checking whether the number of people added to a
conversation was strictly less than the limit - 1, which was causing
conversation with maximum size to be rejected.

This was introduced by the refactoring in #1801.

Author: pcapriotti

services/galley/src/Galley/Validation.hs

@@ -50,7 +50,7 @@ checkedConvSize x = do
   o <- view options
   let minV :: Integer = 0
       limit = o ^. optSettings . setMaxConvSize - 1
-  if length x < fromIntegral limit
+  if length x <= fromIntegral limit
     then return (ConvSizeChecked x)
     else throwErr (errorMsg minV limit "")