fix: a temporary master password is shown once

[blaipr]

TemporaryMasterPass::create() puts the generated key into the issuing administrator's
session, so that the page rendered after the generating request can show it. There is
nowhere else to keep it for that one hop — it is deliberately never persisted in
plaintext, only its bcrypt hash and a blob that is useless without it.

Nothing ever removed it again. There was no clear to call: the whole codebase has exactly
one set and one get for tempmasterpass. And ConfigManager\IndexController reads it on
every load of the Configuration page — getGridTabs() builds every tab the user can
reach in one pass, so it does not even depend on which tab is being viewed. So a value
meant to be shown once was rendered into the HTML again on every later visit, for the life
of that session: after it had expired, after its recipients had used it, and after the
master password had been rotated, none of which touches the session copy.

Reading it now takes it. The method is takeTemporaryMasterPass() rather than a getter
with a surprise in it, since the name is the only thing that stops the next caller
assuming otherwise — and there is exactly one caller.

What this does not change: the value is one global secret for the instance, and the
group picker on the issuing form chooses who it is mailed to, not who may use it —
checkKey() takes no user and LoginMasterPass::loadTemporary() checks none. That is
sysPass's design, one shared master password with a per-user wrapped copy, not a missing
check.

Checked by dropping the clear: the new test fails on the second read.