Drop support 2.7/2.8 and add support for CSRF Token

[pdelboca]

This PR deprecates support for 2.7 and 2.8 to match our policy: https://docs.ckan.org/en/latest/maintaining/releases.html#supported-versions

In Addition:

• It adds support for CSRF token
• It fixes a small bug in the definition of pages_index endpoint. (https://github.com/ckan/ckanext-pages/pull/123/commits/2775384e32605202ba97447e19858e68a47dc062)
• It also deprecates the helper get_plus_icon() in favour of simplicity.
• Refactor back the logic to the plugins.py file since Pylons compatibility is no longer required.
• It renames get_wysiwyg_editor to pages_get_wysiwyg_editor to avoid name clashes.
• Add some tests and 💟