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.

The goal of this component is abstracting away
the difference between metadata fields which are
strings and the ones which are arrays of strings.

This removes the <br> tags which were
previously inserted after every five elements.
Line breaks should be handled dynamically
by the browser.

This makes displaying metadata of an exercise
less verbose. Styling is a bit lacking as of now.

Restructure metadata translations of lists
to make declarations more concise.

Exercises should not depend on search,
so they get their own module.

This is done as to reduce the file size
and complexity of the exercise-details component.

zweimal (and some Test improvments)

Remove unused variable

See merge request sharing/codeability-sharing-platform!34

Resolve "Teaser Content as Word Cloud"

See merge request sharing/codeability-sharing-platform!35

106-teaser-content-as-word-cloud

Conflicts:
	src/main/java/at/ac/uibk/gitsearch/service/dto/SearchResultsDTO.java

Resolve "Die Klassen Person und GitProject gibt es zweimal"

See merge request sharing/codeability-sharing-platform!32

# Conflicts:
#   src/main/webapp/i18n/de/exercise.json
#   src/main/webapp/i18n/en/exercise.json

Fehlermeldung download

See merge request sharing/codeability-sharing-platform!33

Fix mailto link

See merge request sharing/codeability-sharing-platform!31

Prior to this change, the image would
become full-width on narrow screens,
which looks awful.