Canonicalize text and update docs of messages using 'loginreqlink'

Since 'loginreqlink' is being used as a part of patchwork messages we
should try to keep the grammar the same in all of its uses.

"You have to log in" / "You must log in" / "Please log in" were all
used with various frequencies; I changed (hopefully) all uses to
"Please log in".

Two messages receiving 'loginreqlink' as a parameter do not currently
use it: 'changeemail-no-info' and 'resetpass-no-info'. This is
intentional, as they are less "positive" than the rest and thus should
not link to the login form.

Updated and cross-referenced qqq of 'loginreqlink' and messages using it.

Change-Id: Ie245486051d6fbc72a596ba825bfd5d9e3a163bf