(bug 34753) Added label for captcha field shown in forms

(bug 34753) Added label for captcha field shown in forms

Every field should have a descriptive label to clarify what is expected
to be entered.

Though users who are using screen readers are not able to retype the
text shown by the captcha image at least they know what the application
is expecting and may get help from other people.

The label text is also added to the translation arrays for English and
German language as well as the descriptive array 'qqq'.

Removed trailing whitespaces and redundant internationalization
according to reviewer's comment.

Removed usage of global $wgOut;

Change-Id: I975dddb60891b162a62e547bb83669c13cb133a1