Create HTML userfile model; when rendering HTML documents, insert them in IFRAME tags

[prioux]

Create a Userfile model for HTML files.

We have the capability to render HTML documents within our model viewers, but to keep their namespaces (IDs, stylesheets etc) separate from the CBRAIN interface itself, they should be put within <iframe> tags.

[MontrealSergiy]

A PR is rebased and resubmitted