Add content filter as valid filter in webapp

Created by: attfarhan

Adds the new content filter as a valid filter in our webapp, meaning it's recognized in our frontend parser, and will be correctly handled in interactive mode.

Also adds it to the interactive mode filter dropdown.

Created by: rvantonder

So, hm, to be robust, the input should always be quoted (in the raw input at least, not necessarily when displaying pill), and thus also string-escaped if needed. I.e., standard escaping for " with \" and \ with \\. And that would need tests.

But I noticed that currently the filters don't do any escaping for repo or file or... So I would say, go ahead with this, but then address the issue around quoting, etc. as a separate issue, whenever that makes sense to do.

Filed #8418 (closed) for the above.

Created by: attfarhan

Ah yeah, good point on the quoting especially. I'll add that