As somebody in a committe, I want to be able to change the banner for that committe without having to create a pr

### Description
Currently you have to create a pr to the hubbit github repository to change the banner of a committe in hubbIT

My suggestion is having a page in hubbIT where you could change the banners for committee's you are in. I image something similar to chalmers.it where you can change the banner from a simple settings page.
This would make it a lot easier for people to change their banners and would decrease the workload for digIT as they no longer have to review and approve pull requests for banner changes:)

### Definition of Done
- [ ] hubbIT has a page to change banners
- [ ] A user can only change the banner of committee's their active in
- [ ] You can only upload banners of the correct proportion/size (150x48px)