Disable the market community by default + add toggle to settings menu

[devos50]

The market community is important for Tribler since it is an integral part of our self-replicating botnet research. However, the market is not actively maintained and therefore, there might be some bugs that could affect Tribler as a whole. As discussed on Slack with @ichorid, we will disable the market community by default. The market should be enabled through a toggle in the settings panel.