Make left sidebar hidable in task view

I don't know if this is already possible right now, but I couldn't figure it out with the docs, so here goes a feature request:

**Is your feature request related to a problem? Please describe.**
Right now, the annotation interface feels a bit crowded for my use case: In my scenario, an end user follows a hyperlink to one specific task which he completes (example URL: `/projects/1/data?tab=1&task=1`). The **left sidebar** really becomes unnecessary then.

**Describe the solution you'd like**
A project setting to hide the left sidebar while in task view. It needs to be a project settings so users don't need to set this up manually each time they annotate.

**Describe alternatives you've considered**
A way to insert CSS that styles the whole interface. This would be a kind of "escape hatch" for many use cases where people want to change the interface in some way. However, it's a bit hacky and might have security implications, so the solution above would be just fine for now.