do not show category drop down when no tags available, fix style, change button background when selected

Merge Request for tag-cloud-configuration for staging

See merge request arsnova/topic-cloud!5

Merge branch 'tag-cloud-administration' into 'staging'

See merge request arsnova/topic-cloud!9

Added Option to Discard Filters and select questions from now on

See merge request arsnova/topic-cloud!7

# Conflicts:
#	src/app/components/shared/shared.module.ts

Keywords to be selected

Closes #3

See merge request arsnova/topic-cloud!8

# Conflicts:
#	src/app/components/shared/header/header.component.html
#	src/app/components/shared/header/header.component.ts
#	src/app/components/shared/tag-cloud/tag-cloud.component.ts