Add admin session view

[laduguer]

Description

To make it easier for the Java Community team, we'll need an administrator view. Here it should be possible to add a next session of DevLabs.

Acceptance criteria

• Overview of all sessions
• Adding/editing (title/date) of a session on this view
• When a session was updated by the admin all topics that are linked to this session should also display the updated
• When a session was added it should be available for users to update their topic's latest session to the new session
• When a session was added by the admin, it should be available for new users to select this session for their new topic.