Data-loading interface

What do you think about removing the text boxes from the data input UI? This step probably doesn't need to support copy/paste, especially if we use the new file formatting (https://github.com/blekhmanlab/compendium_website/issues/39). We could have:

* File upload prompt – ASV table (samples x ASVs)
* File upload prompt – taxa x ASVs
* File upload prompt (optional) – sample metadata
* Drop-down for selecting example dataset
* Ran suggested offering multiple options here—they would all be formatted in the same way as the user data, if that's helpful, but I'm still processing the raw data (https://github.com/blekhmanlab/compendium_website/issues/36). Maybe positioning this next to the three prompts would indicate this would replace those files with pre-made ones?