Add ability to disable built-in CRX settings

[vsalvino]

Building on the work from #623

See settings:

• CRX_DISABLE_LAYOUT (not recommended)
• CRX_DISABLE_ANALYTICS
• CRX_DISABLE_NAVBAR
• CRX_DISABLE_FOOTER

The plan, based on what @jlchilders11 and I discussed, is to disable navbar and footer in the pro template, and provide a simple boilerplate navbar and footer for customization.

Note: template filters crx_settings and django_settings have been removed, and replaced with tag {% django_setting "MY_SETTING" %}.

[vsalvino]

@michaelsteigman what do you think of these additions?

[michaelsteigman]

works for us! thanks!