Multiple issues with parsing received emails and storing it in db, then displaying it

Description of the issue

As our company will heavily use emailing feature in frappe, we went for hard-testing of frappe emailing. Currently we found multiple bugs and fixed them, but there is one more and bigger issue. That is, with parsing, storing and then showing the emails. In addition, there is still issue with encoding outgoing emails and decoding subjects/sender. The issue list it:

• Incorrectly storing email when it includes application/ld+json type (should not include)
• Completely destroying, destructing and deformatting incoming emails
• Showing email structure incorrectly/not-formatted
• Storing email in db in not escaped way in some cases

Why not to create separate issues?

• I know there is rule, to create separate tickets for each issue, but tbh it would be almost impossible to not to solve one issue with other, because the code for parsing is at the same place of every other issue. Because storing in db and then showing email is in old and bad condition (not correct parsing html, escaping, displaying), it is required to have it refactored and updated (also for better security).
• In the future, it should be good to allow user to decide if he want to enable/disable external scripts/sources for better security

Resolution?

• Me with our team, decided that we will solve this issue and propose a fix and refactoring. Will mention created PR.

Context information (for bug reports)

Output of bench version

frappe 15.8.1

Steps to reproduce the issue

1. Receive email and observe

Observed results screenshots

• Issue: White text on white background:

  

• Issue: Incorrect position of elements

  

• Issue: Extreme big and inconsistent spaces <img width="1512" alt="Image" src="https://github.c