Improved experiment upload predictability

There are two things which make experiment uploading kind of unpredictable/unintuitive:

- It's not always obvious that the **upload experiment** button does requires that a file be chosen first.

- An error occurs when an experiment is uploaded before an experiment is saved for the first time (that is, when an experiment is being added).

To fix this, I've made these changes:

- Removed **upload experiment** button and made experiment upload occur as soon as a file is chosen

- Changed upload_experiment so that it creates and saves a new experiment with a placeholder name and the first user as the experimenter when uploading to an unsaved experiment.