Add singular to make translation easier

Added singular case to two messages (which can never be singular)
so as to facilitate translation. Affected messags:
* massmessage-summary-addmulti
* massmessage-summary-removemulti

Per https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Massmessage-summary-removemulti/sv

Change-Id: I4abe55d6709d8b266442e9e5d93e459bceae869d