Previously, email addresses that weren't connected to a Zulip account
were ignored but now they receive an email stating their email isn't
connected to a Zulip account.
Also, removes the "Thanks for using Zulip!" line at the end of the
find accounts email that's sent when a Zulip account is found.
Updates the i18n test that used this string with another in the
German translation from this a successful account found email.
Fixes part of #3128
Co-authored-by: Lauryn Menard <lauryn@zulip.com>
5410df2a7b