Explicitly set font in HTML e-mails

The e-mails sent by GitLab are in HTML format but don't set any font to use. MUAs can then decide what font to use to render the message content. Unfortunately, Microsoft Outlook will always use Times New Roman in this case and it's impossible to change that. While one could argue that changing the font would be in the responsibility of Outlook, let's face the fact that Microsoft is probably unable to ever change this. Also, it can't hurt to define a font so e-mails are displayed in a predictable way. (At least this is done for 99.9 % of all websites for the same reason: The user doesn't want to care but still doesn't want it to look ugly.) Some areas like coloured headers already set the "Arial" font, so this should probably be set on the entire message level as well to be consistent.