Add Buttons for default/dark/light Theme
Add preliminary dark and light themes
Todo: only the button color itself changes, not the whole theme

Resolve "master is not compiling"

Closes #169

See merge request swtp-block-ws17/arsnova-angular-frontend!103

Resolve "Remove short ID from room creation/room edit"

Closes #140, #141, and #142

See merge request swtp-block-ws17/arsnova-angular-frontend!100

Resolve "navigation (main pages)"

Closes #107

See merge request swtp-block-ws17/arsnova-angular-frontend!101

Resolve "Differentiate between guests and registered users"

Closes #166

See merge request swtp-block-ws17/arsnova-angular-frontend!99

Resolve "navigation (login/logout)"

Closes #108

See merge request swtp-block-ws17/arsnova-angular-frontend!98

Fix some minor style issues related to room create/room edit

Resolve "Refactor project structure"

Closes #74

See merge request swtp-block-ws17/arsnova-angular-frontend!97

Rename identifiers

Rename models

Remove content-create