htmlutils: improve js string escaping

• Moves/renames jsonutils.wash_for_js() function to htmlutils.escape_javascript_string() as this function would mostly be used to escape javascript in HTML context. Updates bfe_bookmark accordingly.

• No longer adds double-quotes around the returned string in order to be more flexible regarding the context in which the string is inserted.

• More complete escaping of Javascript chars when 'json' is not installed.

• Adds parameters to control the level of escaping, to be adjusted according to the context (HTML vs JS output, XHTML vs HTML, content-type, use of CDATA delimiters).

• Adds docstrings and unit tests.

• Merge note: fixed indentation in htmlutils.py.

Tested-by: Tibor Simko <tibor.simko@cern.ch>