Minor cleanup following sprint 1

[apdavison]

todo:

• use icons only from Material UI, it will be confusing if we mix icons from two different sources
• remove commented-out code
• remove unused imports
• use Accordion in place of ExpansionPanel (here it says "⚠️ The ExpansionPanel component was renamed to Accordion to use a more common naming convention")
• move API base URL (https://nmpi.hbpneuromorphic.eu/api/v2/) to a globals.js file
• make date formatting a function and move it to a utils.js file
• finish removing bootstrap