Alter Email Message Default Policy to Comply With RFC 5322

[Waghabond]

The Python email library's default policy does not fully comply with the Internet mail standard.
So this will alter the default policy when it is used by redmail so that the email messages it creates will comply with the internet standards and be more likely to be displayed correctly by all email clients.

[codecov-commenter]

Codecov Report

Base: 99.68% // Head: 99.68% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (a9f77b2) compared to base (a813ac1).
Patch coverage: 100.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master      #75   +/-   ##
=======================================
  Coverage   99.68%   99.68%           
=======================================
  Files          13       13           
  Lines         627      636    +9     
=======================================
+ Hits          625      634    +9     
  Misses          2        2           

Impacted Files	Coverage Δ
redmail/email/body.py	100.00% <100.00%> (ø)
redmail/email/sender.py	100.00% <100.00%> (ø)
redmail/email/utils.py	100.00% <100.00%> (ø)

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[Miksus]

Thanks for fixing this! You are indeed correct:

2.3. Body
The body of a message is simply lines of US-ASCII characters. The
only two limitations on the body are as follows:
o CR and LF MUST only occur together as CRLF; they MUST NOT appear
independently in the body.

(https://www.rfc-editor.org/rfc/rfc5322)

I'll add to the versions.rst about this later today or tomorrow and then I think we can merge this.

[Miksus]

Whops, it seems I spaghettified the PR by working with the same workspace as with the other PR...

But I think this is good.