Clean up the HFNC dashboard

[ethanjli]

This PR closes #309 by changing the layout of the HFNC-only dashboard (removing plots, and showing both set and measured values for FiO2 and flow rate) and increasing the size of text on the dashboard. Additionally, in preparation for the v0.7 release, it:

• Disables the screensaver
• Reenables the FDO2 sensor in the firmware by default
• Fixes unintuitive behavior between the set alarms screen and the "Start Ventilation" button in standby mode (see https://github.com/pez-globo/pufferfish-software/issues/324#issuecomment-825985574 for more details)
• Fixes parts of #307
• Adds an emergency use notice to the landing screen

[ethanjli]

Because this PR has been sufficiently (though lightly) tested on basic functionality and is completes the v0.7 milestone, I'm going to merge this into develop as-is, and as prototype-grade code. This version of software should be tested more thoroughly by other people before any deployment. Any serious bugs which need to be fixed on v0.7 will be addressed in hotfix branches and merged onto develop in future PRs. For records-keeping:

1. This project is licensed under Apache License v2.0 for any software, and Solderpad Hardware License v2.1 for any hardware - do you agree that your contributions to this project will be under these licenses, too? Yes
2. Were any of these contributions also part of work you did for an employer or a client? No
3. Does this work include, or is it based on, any third-party work which you did not create? No