Avoid very long log lines and resulting OOM

[rfc2822]

Purpose

Sometimes data like a vCard with a big (16 MB) photo causes very long log lines (as in: 40 MB string), which may cause out-of-memory conditions.

Depends on bitfireAT/vcard4android#46

Short description

This PR tries to avoid long log lines on two layers:

1. The original cause in vcard4android. Contact.toString: truncate note, photo, unknown properties vcard4android#46 reduces Contact.toString() size if there are big photos or long notes or unknown properties.
2. PlainTextFormatter now has some truncation logic to avoid far too long log lines.

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[github-actions]

This PR/issue depends on:

• Contact.toString: truncate note, photo, unknown properties vcard4android#46

[sunkup]

No test? Works fine though 👍

[rfc2822]

No test?

You're right ^^

[rfc2822]

Added test.