LEARNER-2977

Fixes

2. Add site configuration overrides to theming/helpers.py
3. Move microsite.get_value from theming/helpers to site_configuration/helpers
4. Move microsite_configuration.microsite.get_value usages to site_configuration.helpers.values

2. Update COMPREHNSIVE_THEME_DIR to COMPREHENSIVE_THEME_DIRS
3. Update paver commands to support multi theme dirs
4. Updating template loaders
5. Add ENABLE_COMPREHENSIVE_THEMING flag to enable or disable theming via settings
6. Update tests
7. Add backward compatibility for COMPREHEHNSIVE_THEME_DIR

There's a lot of useful things in PR #8271 that provide a framework
for the comprehensive theming system. If we need to remove the edx.org
theme from the codebase, we can leave most of PR #8271 in, to make it
easier to build on top of and get this feature back in.

There's a lot of useful things in PR #8271 that provide a framework
for the comprehensive theming system. If we need to remove the edx.org
theme from the codebase, we can leave most of PR #8271 in, to make it
easier to build on top of and get this feature back in.

This is a squash of 38 commits ending with
5b080f979d692804452400ac5bed9b17c50b001e