Fixes #3574: Incorporate bugbug ml labelling into moderation process

[ksy36]

In this PR I've added functionality to automatically close issues marked by bugbug as invalid.

1. Issues marked by bugbug as invalid moved to "ml-autoclosed" milestone.
2. Once milestone event is received, public issue content is replaced with a placeholder text and label "bugbug-probability-high" is added, public issue closed.
3. Private issue closed

Discussed in #3574

Example of a public test issue that was closed using this code: webcompat/webcompat-tests#2638

[ksy36]

r? @karlcow

[karlcow]

@ksy36 This is really really really cool.

That will probably help a lot the softvision team. I made some comments and suggestions.

Only two points to think about:

1. Milestones fetching vs hardcoded
2. Logging success and failures of communicating with bugbug

[ksy36]

Ready for another look @karlcow :)

[karlcow]

Thanks a lot @ksy36
Awesome work.

[ksy36]

Thanks!