[WIP] Make reviewer/decision email copy more empathetic and constructive

[bacharakis]

Update developer-, user-, and reporter-facing email templates to use a warmer, more respectful and constructive tone while keeping the required actions clear and the policy/legal/appeal content intact.

Copy sourced from the "addons-server emails" review doc. Changes are scoped to the specific wording that needed updating to keep diffs easy to review. Note: templates wrapping copy in {% blocktrans %} will need re-localization in Pontoon.

Fixes mozilla/addons#ISSUENUM

Description

Context

Testing

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.

[wagnerand-moz]

I went over some of the changes with Christos over zoom. The general feedback theme that emerged was to consider making the texts less casual and more professional without sacrificing the constructive tone.

Before merging, we'd likely need to do a thorough accuracy review, as I spotted some additions that are unactionable or impossible, for example in the user-deleted email.