Modernize activation email

[int-y1]

Closes #560. Supersedes #1916.

New subject: Activate your DMOJ account

New .txt:

Please activate your DMOJ: Modern Online Judge account in the next 7 days.

Please click on the following link to activate your account:
    http://localhost:8081/accounts/activate/9f84bf2f3c03025e3f302d1e3580351c3ad6d57d4e79c89df29fd8c773e67cfa/

Alternatively, you can reply to this message to activate your account. Your reply must keep the following text intact for this to work:

9f84bf2f3c03025e3f302d1e3580351c3ad6d57d4e79c89df29fd8c773e67cfa

New email:

<div style="border:2px solid #fd0;margin:4px 0;"><div style="background:#000;border:6px solid #000;">
<a href="http://localhost:8081"><img src="http://localhost:8081/static/icons/logo.svg" alt="DMOJ" width="160" height="44"></a>
</div></div>

<div style="border:2px solid #337ab7;margin:4px 0;"><div style="background:#fafafa;border:12px solid #fafafa;font-family:segoe ui,lucida grande,Arial,sans-serif;font-size:14px;">
<strong>Thanks for registering on the DMOJ! We&#x27;re glad to have you.</strong>
<br><br>
The last step is activating your account. Please activate your DMOJ account in the next 7 days.<br><br>
Please click on the following link to activate your account:
<br>
<a href="http://localhost:8081/accounts/activate/9f84bf2f3c03025e3f302d1e3580351c3ad6d57d4e79c89df29fd8c773e67cfa/">http://localhost:8081/accounts/activate/9f84bf2f3c03025e3f302d1e3580351c3ad6d57d4e79c89df29fd8c773e67cfa/</a>
<br><br>
Alternatively, you can reply to this message to activate your account. Your reply must keep the following text intact for this to work:
<p style="font-family:monospace;">9f84bf2f3c03025e3f302d1e3580351c3ad6d57d4e79c89df29fd8c773e67cfa</p>
See you soon! If you have problems activating your account, feel free to send us an email at <a href="mailto:admin@example.com">admin@example.com</a>.
</div></div>

Email looks the same in Outlook.

[Xyene]

LGTM, thanks!