Make two buttons for dataset- and participant-level results

[surchs]

Originally posted by @michellewang in neurobagel/query-tool#93 (comment):

I didn’t realize until someone pointed it out yesterday that we could toggle between dataset-level and participant-level results. Personally, I think it would be better to have 2 download buttons instead of the “Toggle level of detail to download” checkbox

To resolve:

• have one permanently visible button "Download dataset results". Clicking it will download the dataset-level.csv
• have one permanently visible button "Download participant results". Clicking it will download the participant-level.csv
• Both buttons should become enabled and disabled in the same way the current download button does (e.g. disabled unless at least one dataset is selected)
• If button text is too large, we can have a linebreak in there (i.e. make button bigger)
• Update README.md
• Update related tests

[rmanaem]

@surchs @jarmoza Do you have any thoughts on the placement of the buttons in the UI, one stacked on top of the other or next to each other?

[jarmoza]

I think side by side.