Resolve "Easy Access to ReadMe.md (and potentially other files)"

See merge request sharing/codeability-sharing-platform!38

Merge remote-tracking branch 'origin/development' into 111-easy-access-to-readme-md-and-potentially-other-files

Dto improvements and additional metadata

See merge request sharing/codeability-sharing-platform!39

111-easy-access-to-readme-md-and-potentially-other-files

Conflicts:
	package-lock.json
	src/main/webapp/app/search/search.module.ts

Refactor exercise metadata display

See merge request sharing/codeability-sharing-platform!36

The exercise metadata component is only rendered
when it gets a value which is not null and has
a length of at least one (either string or array).
If it gets a value which is translated before it
is being passed, the result will be something of
the form "translation-not-found[...]", which has
a length of at least one, so it is rendered.
To prevent this, check if a value is set before
translating it.

Frontend refactoring

See merge request sharing/codeability-sharing-platform!29

Make close button of exercise details less prominent

See merge request sharing/codeability-sharing-platform!28

They were replaced with exercise-metadata-item.