[DRE-maint] Bug#915050: Keep out of testing

[Moritz Muehlenhoff]

Source: gitlab
Severity: serious

Gitlab is too fast-moving with weekly releases and backporting security fixes has already
failed us for stretch, keep it out of testing.

To meaningfully support it for use on stable, this would require some of the
infrastructure/policy changes discussed in the "What can Debian do to provide
complex applications to its users" thread from earlier the year on debian-devel.

Cheers,
        Moritz