Limit level of threads

[salsa-nathan]

Purpose

This pull request adds the ability to limit the number of comment threads to display and can be commented on further.

Approach

• Adds a configuration option to the CKAN .ini file (ckan.comments.maximum_thread_depth) to restrict the number of threads comments can extend to.
• Controls the thread depth limit on the front-end based on the limit set in the CKAN .ini configuration file, e.g. ckan.comments.maximum_thread_depth = 3

Other items addressed in approach:

• Added auth changes submitted in PR 1 (Updating to handle new_authz renaming to authz.py #1)
• lxml 4.4.1 added to requirements to satisfy installation of the extension
• Basic front-end styling of comments to indent replies