Increase timeout for live docs

Note: it seems that this setting 'idle_timeout' is only used for live
documentation browser pages, so it should be safe to change it without
affecting users' worksheets' behavior.
Make separate config var for pub timeout

Note: the commit message of the parent of this commit is incorrect. The
timeout variable did control all worksheets, hence this commit.
Rearrange config variable descriptions
"Remember me" has never worked and is not linked to anything.
Checked as far back as sage-4.7. Removing.
Adapt to new Flask extension module schema

Naming your Flask extension module "flaskext.foo" is now discouraged in
favor of naming it "flask_foo". To accommodate both styles, Flask 0.8
and higher allows you to import "flask.ext.foo", which will first try
"flask_foo" and then try "flaskext.foo". We should do this for all our
flask extension imports.

In the short term, this allows sagenb to build against the latest
version of Flask-AutoIndex, i.e. it deals with the following commit:
https://github.com/sublee/flask-autoindex/commit/2138855d69a124cdaad
Merge pull request #148 from ppurka/rm_remember

"Remember me" has never worked and is not linked to anything.
Merge pull request #149 from kini/flaskext

Adapt to new Flask extension module schema
pub_timeout is useless

Thanks to @ppurka for pointing out that published worksheets are never
interactive anyway.
add options for live documentation timeout to the notebook command
leave doc_timeout to the default value from server_conf.py
increase timeout for live documents to 600s
Merge pull request #1 from ppurka/livedoc_timeout

add options for live documentation timeout to the notebook command
Merge pull request #147 from kini/livedoc_timeout

Increase timeout for live docs