[BuildStream] Minor gitlab permission changes

[Tristan Van Berkom <tristan vanberkom codethink co uk>]

Hi all,

This is just an explanation on the list in case people received
notifications of permission changes, we made some minor changes today.

Background:

  A couple weeks ago we handed out merge rights to anyone who
  has contributed in the past and landed patches.

  We did this pretty blindly by setting all of these users
  as "Maintainers" (my mistake).

The correction:

  Since being a "Maintainer" also opens up a bunch of additional
  permissions to modify CI and access some project related keys, this
  needed to be fixed.

  Today I have done the following:

    o Added explicit merge permission to everyone I had previously set
      to have the "Maintainer" role
    o Set this same group of contributors back as "Developer"

The net effect is basically that if you were given the "Maintainer"
role so that you can have merge power, and this has been adjusted back
to "Developer", don't worry; you you still have merge powers :)

If this has broken for anyone (there is a list of over 20 contributors,
but I don't think I missed anyone), then please let me know and I will
correct it !

Cheers,
    -Tristan