refactored statistics so that they are loaded by the statistics service and then resolved the collections bug

added security check that checks if user can see the exercise he is liking and added an index on exercise_id to likes table

Resolve "Reduce Usage of GitLab Groups in GitSearch"

See merge request sharing/codeability-sharing-platform!57

thumbs_up button implementation

See merge request sharing/codeability-sharing-platform!54

Merge branch 'thumbs_up' of https://sharing-codeability.uibk.ac.at/sharing/codeability-sharing-platform into thumbs_up

against unauthorized users

against unauthorized users

Conflicts:
	.yo-rc.json
	package-lock.json
	src/main/resources/config/liquibase/master.xml
	src/main/webapp/app/exercise/exercise-details/exercise-details.component.html
	src/main/webapp/app/exercise/exercise-details/exercise-details.component.ts
	src/main/webapp/app/search/search.component.ts
	src/main/webapp/i18n/de/global.json