RFR: 2610: Mailman 3 support

[Erik Joelsson]

The OpenJDK mail archive will move to Mailman 3 at some point in the future. To prepare for that, Skara needs to be made Mailman 3 compatible, specifically for reading emails from the archive to be able to publish them as comments in PRs. There are two major changes

### 1. REST API
The REST API for reading mbox archives is different. It's a lot better in Mailman3. Instead of having to read in fixed monthly chunks, we can request a custom date interval. The data is also returned gzipped instead of in plain text. To retain backwards compatibility, I chose to implement the Mailman 2 and 3 support as subclasses with some shared implementation. This caused a lot of mechanical changes in tests, just updating method names or signatures since the API for creating a Mailman server object changes. It also forced some cleanup in bots where MailmanServer objects were created unnecessarily. I updated some tests to use the Mailman3 implementation where possible to exercise the new code more. 

The new implementation is somewhat taking advantage of the new capabilities in the API, but it could maybe be done more efficiently when polling for updates. 

### 2. Mbox format
The format of the mail bodies in the returned mbox format is quite different. It's now MIME encoded and can be encoded in a few different ways ("7bit", "8bit" and "quoted-printable" have been observed so far). I've tried to implement support for decoding all the ways I've so far observed.

Some examples of this patch in action can be found in https://github.com/openjdk/playground/pull/246. Note that the earlier comments have problems that eventually got resolved as I ironed out the details. 

An observation I've made is that the new server introduces hard line breaks in emails as they are archived. Those are visible already when browsing the archive. I'm not sure if this is a setting in Mailman itself or not. Those line breaks are not part of the email I received when subscribing to a list, just in the archive, and so also in any comments posted by Skara.

This is a big patch, so sorry in advance.

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Commit messages:
 - Fix whitespace
 - Better query strategy for mailman3
 - cleanup
 - Decode emails bodies
 - Parse new mbox format
 - MailingListNotifier config does not need archive
 - Bot factory support for different archive types
 - Tests are passing
 - Refactored TestMailmanServer to prepare for V3 variant

Changes: https://git.openjdk.org/skara/pull/1743/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1743&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2610
  Stats: 1449 lines in 23 files changed: 1059 ins; 233 del; 157 mod
  Patch: https://git.openjdk.org/skara/pull/1743.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1743/head:pull/1743

PR: https://git.openjdk.org/skara/pull/1743