Add twig templating

What does this MR do?

Introduces twig templating from two perspectives:

• the global layouts (the things in tpl/)
• individual components (currently just rendering the topbar on the dashboard)

It's quite easy to implement more individual components without having much impact on the rest of the site (can just use the normal addContent function).

At some point it would be a good idea to render whole pages (e.g. dashboard) via a template that extends one of the layouts, but does not need to happen right now.

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

Hmmm, I wouldn't be surprised actually if something breaks a little, but all tests pass and I can click around the site ok. Would be good to have people try it out on the beta site.

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))