Add a canceller role to the TimelockController

[Amxx]

Fixes #2980

PR Checklist

• Tests
• Documentation
• Changelog entry

[frangio]

"Cancellor" is a typo, it should be "canceller".

In the changelog, we should explain that if an instance is upgraded the role needs to be set up (admin and members).

[Amxx]

AFAIK, both cancellor and canceller are valid. One is American english, the other is British english.

[frangio]

No, you may be confusing it with "chancellor"? "Cancellor" doesn't seem to exist at all.

[Amxx]

I did put "Cancellor" in my translator, and got what I expected.

I'm going to change it just to be safe.

(and no, I'm not confusing chancellor Palpatine with cancellor Palpatine :P)

[frangio]

I would expect the tests to set up different proposer and canceller accounts, so we can separately test them. Note: it also makes sense to test that in the default constructor the proposer array is assigned canceller roles.

[frangio]

This is good but needs a changelog entry.