Progress Bar on Image Upload while Adding/Editing an Event

[danieldanielecki]

While working on #38, there was no visual implementation of progress for the image being uploaded, and if the user adds an event too quickly, the image needs to be uploaded.

Most cases should be fixed by #54, but only for good-quality networks. That's why we should have a progress bar to disable the possibility of adding/editing an event until the image is added. It could've been combined with #9.

https://callstack.github.io/react-native-paper/docs/components/ProgressBar/ should suffice.