distros.conf: add *updates, *backports, *proposed for jessie

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p.conf.anbe: update various limits

Signed-off-by: Andreas Beckmann <anbe@debian.org>
add changelog entry for scripts updates

Signed-off-by: Andreas Beckmann <anbe@debian.org>
add trailing colons to bug template subjects to encourage giving more details

Signed-off-by: Andreas Beckmann <anbe@debian.org>
add fails_to_upgrade_from_jessie* bug templates

Signed-off-by: Andreas Beckmann <anbe@debian.org>
distros.conf: switch 'testing' to 'stretch', deactivate it for stable

deactivate it to avoid having a possibly wrong hardcoded definition of
'testing' in 'stable'

the *only* effect is that piuparts will use a separate basetgz for 'testing'
and not reuse a 'codename'd one

Signed-off-by: Andreas Beckmann <anbe@debian.org>