CRUD-ify categories

[wayne-li2]

Food pantry volunteers should:

• Create, update, and delete categories from a category table.
• This includes uploading images to Firebase Cloud Storage
• The mobile app should, on startup, download all images and retrieve the categories from the database, and display them.
• While the database call must succeed (to get all the categories), the images are optional and should be placeholders until the call to Cloud Storage completes.
• The Cloud Storage call failing should not affect the usability of the app.

[wayne-li2]

This is probably not a good issue as it is too broad. We will break this apart as we go on (already started).