Improve password reset email

[int-y1]

Make sure that SITE_ADMIN_EMAIL gets into the template. The new email looks like this:

<span style="display:block;margin: 0 2px;padding: 1em;border: 3px solid #FFDE05;background-color: #000;border-radius: 6px;font-size: .95em;color: #444;margin-bottom:0.75em">
<div style="display:table;margin-left:2em"><img src="https://avatars1.githubusercontent.com/u/6934864?v=3&s=101" style="display:inline;vertical-align: middle">
<h1 style="font-size:4em;display:inline;vertical-align: middle"><a href="http://localhost:8081" style="text-decoration:none;color:gray"><span style="color: #FFDE05">DM::</span>OJ</a>
      </h1>
</div>
</span>
<div style="display:block;margin: 0 2px;padding: 1em;border: 3px solid #2980B9;background-color: #f8f8f8;border-radius: 6px;font-size: .95em;color: #444;">

<b>Forgot your password on the DMOJ: Modern Online Judge? Don&#39;t worry!</b><br><br>
To reset the password for your account &quot;admin&quot;, click the button below.
<p align="center">
<a href="http://localhost:8081/accounts/password/reset/confirm/Mg-61r-c918e2d258427a7110d8/" style="cursor: pointer;display:block;text-align: center;padding: 4px 2px 5px;color: white;border: 1px solid #666;border-radius: 1px;background: #2980b9;background: linear-gradient(180deg, #00aee0, #2980b9);text-decoration: none;line-height:2em;font-size:1em;width:12em;">Reset Password</a>
</p>
See you soon! If you have problems resetting your password, feel free to shoot us an email at <a href="mailto:admin@example.com">admin@example.com</a>.
</div>

[codecov-commenter]

Codecov Report

Merging #1945 (86ee074) into master (966b37d) will increase coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #1945      +/-   ##
==========================================
+ Coverage   46.46%   46.47%   +0.01%     
==========================================
  Files         236      236              
  Lines       13114    13118       +4     
==========================================
+ Hits         6093     6097       +4     
  Misses       7021     7021              

Impacted Files	Coverage Δ
dmoj/urls.py	89.47% <ø> (ø)
judge/views/user.py	39.20% <80.00%> (+0.74%)	⬆️

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[Xyene]

LGTM, thanks!