forum rework

What does this MR do?

• forum remade in vue
• Forum API
• adds a emoji component
• adds vue i18n filter
• adds vue media query mixin
• reactions
• vue date filter
• js api abstraction
• vue-wrapper improvements
• enable markdown markup for forum posts
• proper handling of forum email notifications

What does this MR not do, but the next one(s)

• tests
• emojis per svg instead of unicode character

How confident are you it won't break things if deployed?

there's too much stuff

Links to related issues

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))