…ption (OCA#58)

* [8.0][IMP][mass_mailing_custom_unsubscribe] Get reasons for unsubscription.

- Imported last updates from v8.
- Adapted to v9.
- Added a saner default to `mass_mailing.salt` configuration parameter by
  reusing `database.secret` if available, hoping that some day
  odoo/odoo#12040 gets merged.
- Updated README.
- Increase security, drop backwards compatibility.
  Security got improved upstream, which would again break compatibility among current addon and future master upstream.
  I choose to break it now and keep it secured future-wise, so I drop the backwards compatibility features.
- Includes tour tests.
- Removes outdated tests.
- Extends the mailing list management form when unsubscriber is a contact.
- Adds a reason form even if he is not.
- Avoids all methods that were not model-agnostic.

[FIX][mass_mailing_custom_unsubscribe] Reasons noupdate

After this fix, when you update the addon, you will not lose your customized reasons.

[FIX] Compatibilize with mass_mailing_partner

Current test code was based on the assumption that the `@api.model` decorator on `create()` ensured an empty recordset when running the method, but that's not true. This was causing an incompatibility betwee these tests and the `mass_mailing_partner` addon, which works assuming 0-1 recordsets.

Now records are created from an empty recordset, and thus tests work everywhere.

Update instructions

If the user does not add the unsubscribe snippet, nothing will happen, so it's added to README to avoid confusion when testing/using the addon.

[FIX] Use the right operator to preserve recordsets order

Using `|=` sorts records at will each time (treating them as Python's `set`).
Using `+=` always appends a record to the end of the set.
Since we are using the record position in the set, this caused the test to work sometimes and fail other times. Now it works always.

* [IMP] mass_mailing_custom_unsubscribe: GDPR compliance

- Record resubscriptions too.
- Record action metadata.
- Make ESLint happy.
- Quick color-based action distinction in tree view.
- Add useful quick groupings.
- Display (un)subscription metadata.
- Pivot & graph views.

This creates inconsistency issues when assembling them in the README.

Currently translated at 80.0% (44 of 55 strings)

Translation: social-11.0/social-11.0-mass_mailing_custom_unsubscribe
Translate-URL: https://translation.odoo-community.org/projects/social-11-0/social-11-0-mass_mailing_custom_unsubscribe/fr/

Fix assertIn error thrown in testing mode

Updated by Update PO files to match POT (msgmerge) hook in Weblate.