Refactor mailing tempalte to be in the GEWIS STIJL

[JustSamuel]

Based on @rinkp his remark on this issue. It might be nice to change the mailing theme.

[rinkp]

When implementing this, it is vital not to change any items in the output. It is known that there are redundant divs, that a lot of styling is applied directly to items instead of classes, and that there is a bunch of extra CSS.
The reason is that emails are hard to work with, and there are a lot of different email clients.
The current styles have been verified to work in Thunderbird (Windows/Linux), Outlook (Windows, Android), and GMail (Web/Android/iOS) 's light and dark modes. Additionally, they have been tested with the Google and Microsoft spamfilters and common defaults for rspamd and SpamAssassin to make sure they are not marked as spam.

Guidelines for usage: don´t include too much HTML in the yaml, don´t make fake hrefs (wrong: <a href="https://gewis.nl/super-long-url">gew.is/short</a>, correct: <a href="https://gew.is/short">https://gew.is/short</a>, also correct: <a href="https://gewis.nl/super-long-url">click here</a>, and be careful with including images.