Use plural forms

[David Planella]

The scope code has these as separate strings:

- %1 new message
- %1 new messages

It'd be best to use plural forms so that they appear in a single string and so it works for languages with multiple plurals.

[GunChleoc (gunchleoc)]

I have identified some more:

%1 members

%1 members, %2 online

Best split this one up into 2 strings - I only need the plural form for "%1 members", but other languages might need it for "%2 online" as well.

[GunChleoc (gunchleoc)]

And another one:

%1 are typing

This string has already been fixed for another occurrence, but at least one is still left that isn't using plural forms.

The correct string ha the translators comments:

TRANSLATORS: Indicates in the chat header who is typing.
TRANSLATORS: Indicates in the chat header which contacts are typing.

And the wrong string has the translators comment:

TRANSLATORS: Indicates in the chat header which contacts are typing.