enable eslint for mail/extensions/openpgp

This patch was made on top of the patches from bug 1616575 and bug 1616558, so those need to go in first.

Also, I need the eslintrc fixes from bug 1616608 to land first, because this patch has a function with big complexity.

Finally, I have one small TODO left, in addition to the attached huge patch, to avoid one more eslint warning. I'll provide a separate, incremental patch for that.

Pushed by kaie@kuix.de:
https://hg.mozilla.org/comm-central/rev/e3303808c596
enable eslint for mail/extensions/openpgp. r=mkmelin
https://hg.mozilla.org/comm-central/rev/507970329f68
Use new syntax for ChromeUtils.import. r=mkmelin
https://hg.mozilla.org/comm-central/rev/6c75755a02eb
Fix a remaining eslint issue, changing indent. r=mkmelin DONTBUILD

Attached another patch to fix:

• a few places where I used "const ... import", which is failing
• signing. With this change, sending signed or signed&encrypted works

The enigmail code used "test signing" to find out what hash algorithm gnupg used for a given signer key. I disabled that, and hardcoded to use SHA256 for now. Added a comment that this needs to be coordinated with rnp.jsm which also hardcodes SHA256 currently.

Pushed by kaie@kuix.de:
https://hg.mozilla.org/comm-central/rev/423c84a5eae7
const/var import fixes, and fix signing. r=PatrickBrunschwig DONTBUILD