[wip] Stylesheets split into separate style guide repo

https://www.openproject.org/work_packages/7630

**BEFORE MERGING**
The developer who reviews this PR will also need to set up their subtree on their repo:
```
git remote add assets https://github.com/finnlabs/openproject-style-guide.git
git rm -r app/assets
git commit -am "removing split app/assets directory"
git subtree add --prefix=app/assets --squash assets master
```

NOTE: The above is only needed for developers who actually need to push/pull from upstream.

To push and pull from upstream (the openproject-style-guide repo):
```
git subtree pull --prefix=app/assets --squash assets master
git subtree push --prefix=app/assets assets master
```