Grammar and consistency in English messages

Made "accounts" plural, because they are separate, and not
one account.

In the merged accounts there's always the current account
and at least one account from another wiki, so it's always plural.

The unmerged accounts can be singular or plural.

Based on changes in wikidata.org by [[User:Riley Huntley]].

Change-Id: If202dea4dfb21bd97c930b8b771e3be107525977
Merge "Grammar and consistency in English messages"