Move Node Version Source to Site Config setting over WEBSITE_NODE_DEFAULT_VERSION

## Backfilling values:
Option 1:
- Copy current app setting and fill into website config, regardless of current website config value

Option 2:
- Copy current app setting and fill into website config, regardless of current website config value
- Delete app setting value? (NOTE: THIS RESTARTS SITE)

Option 3:
- Explicitly null all website config values (wait until complete for all sites)
- Copy current app setting and fill into website config

## Updating from Portal, CLI, ARM:
**Input**
(If both, website_config takes priority.)
- if (website_config): take website_config value
- else if (app_setting): take app_setting value

**Action**
Option 1:
- Save as both app setting and website config

Option 2 & 3:
- Save as website config and actively remove app setting value

## Reading node version for site start
Option 1:
- Read app setting

Option 2:
- if (app_setting): read app setting, because not yet updated
- else if (website_config): read website config

Option 3:
- if (website_config): read website config
- else if (app_setting): read app setting

No files, tests, or entry points are named. First resolve which backfilling, update, and startup-reading option is intended, then trace the Portal, CLI, ARM, and site-start configuration paths. Done means the selected migration behavior is implemented consistently and the handling of WEBSITE_NODE_DEFAULT_VERSION and website config is specified for each path.

Written by the indexing model from the issue text.