[Merge] ~hyask/ubuntu-dev-tools:skia/i-b-f-d_multipart into ubuntu-dev-tools:main

[Skia]

The proposal to merge ~hyask/ubuntu-dev-tools:skia/i-b-f-d_multipart into ubuntu-dev-tools:main has been updated.

Description changed to:

This MP tried its best to take into account feedback from previous ones, in the hope to finally get the situation fixed:
https://code.launchpad.net/~ubuntu-dev/ubuntu-dev-tools/+git/ubuntu-dev-tools/+merge/406679
https://code.launchpad.net/~schopin/ubuntu-dev-tools/+git/ubuntu-dev-tools/+merge/407188

This is ready for a first round of review to get feedback on the approach, ~~but not ready to merge yet: at least the attachment creation part needs testing when staging Launchpad will be back in shape.~~

EDIT: now fully tested against qastaging Launchpad. See those two imported bugs:
https://bugs.qastaging.launchpad.net/ubuntu/+source/twisted/+bug/2043832
https://bugs.qastaging.launchpad.net/ubuntu/+source/fontconfig/+bug/2043829

For more details, see:
https://code.launchpad.net/~hyask/ubuntu-dev-tools/+git/ubuntu-dev-tools/+merge/468091
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