Move HTML reports to subject or session folder

[tsalo]

Doing this will make subject- or session-wise data more easily shareable. A few ideas:

1. Create a new flag to determine the location of the HTML report.
2. Move the figures folder into the session folder.
3. Create an index HTML file at the top level to make it easier to open subject- or session-wise reports without having to navigate to each folder.

[tsalo]

In QSIPrep, we're planning to add a --report-output-level parameter that accepts a value of root, subject, or session, with the default being root. See https://github.com/PennLINC/qsiprep/issues/848.