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

[Skia]

Skia has proposed merging ~hyask/ubuntu-dev-tools:skia/i-b-f-d_multipart into ubuntu-dev-tools:main.

Commit message:
import-bug-from-debian: handle multipart messages

With multipart messages, like #1073996, `import-bug-from-debian` would
produce bug description with this:
```
[<email.message.Message object at 0x7fbe14096fa0>, <email.message.Message object at 0x7fbe15143820>]
```
For that kind of bug, it now produces a correct description with the
plain text parts concatenated in the description, the attachments added
as attachments, and the inline images converted to attachments with an
inline message placeholder.

See #981577 for a particularly weird case now gracefully handled.
If something weirder happens, then the tool will now abort with a clear
message instead of producing garbage.

Requested reviews:
  Mattia Rizzolo (mapreri)

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

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.
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