Elegantly handle missing images in HTML report

[tsalo]

Summary

Some figures won't be generated, depending on the settings. For example, in the 4-echo manual selection, we won't have T2* or S0 figures:

Next Steps

1. Replace string.Template HTML templating with Jinja2.

[tsalo]

By the way, Jinja2 is a dependency of bokeh, so we already require it implicitly.