Session Handing / Sessiongrid

[artifactdev]

Add component which shows the sessiongrid like in the old website (also responsiveness)

1. Add session-service which provides the sessions with mocked data how pretalx provides it with their api
2. add the interfaces accordingly (like done for speakers)
3. add information of the speakers session to the speaker detail page
4. add a component which shows all sessions like described at the top

Please put special focus on the interfaces that they are logically mapped with the information we need.