Move extensions marketplace settings to webide domain

Issue: Move extensions marketplace settings to webide ... (#472643 - closed)

What does this MR do and why?

• Moves extensions marketplace settings to webide domain.
• Adds independent version of settings module to webide domain
• Includes associated refactorings to extract out common logic from settings module
• Includes support for only processing requested settings

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Working Extensions Marketplace in Web IDE ("boxes" in left nav):

How to set up and validate locally

Relevant flags and setting

• web_ide_extensions_marketplace feature flag enable/disable:
  • Feature.enable(:web_ide_extensions_marketplace)
  • Feature.disable(:web_ide_extensions_marketplace)
• web_ide_oauth feature flag enable/disable:
  • Feature.enable(:web_ide_oauth)
  • Feature.disable(:web_ide_oauth)
• User Preferences -> Enable extensions marketplace (https://docs.gitlab.com/ee/user/profile/preferences.html#integrate-with-the-extension-marketplace):

General validation

• Verify that all extension marketplace settings / metadata are behaving the same as master for both Web IDE and workspaces
• Verify that existing workspaces settings still behave the same

Specific validation steps

1. Ensure vscode_web_ide flag is enabled (it is default-enabled, should be true unless you've explicitly disabled).
2. With web_ide_oauth and web_ide_extensions_marketplace flags on and enable Enable extensions marketplace in User Preferences (and click save), open Web IDE (Edit -> Web IDE from a project page) and extensions marketplace should be available (see screenshot above).
   1. Try to install an extension.
   2. Restart the Web IDE.
   3. Ensure that extension persists.
3. Disable web_ide_oauth flag, re-open Web IDE
   1. extension should no longer persist, marketplace should be unavailable.
4. Re-enable web_ide_oauth flag and disable web_ide_extensions_marketplace flag, re-open Web IDE
   1. extension should no longer persist, marketplace should be unavailable.
5. Re-enable web_ide_extensions_marketplace, keep web_ide_oauth flag enabled. DISABLE Enable extensions marketplace in User Preferences (and click save). Re-open Web IDE
   1. Extension marketplace is "enabled" (clickable), but displays "No extensions found"
6. Re-enable Enable extensions marketplace in User Preferences (and click save), re-open Web IDE:
   1. Extension marketplace is enabled, extensions are displayed (but previously installed extension must be re-installed after previous disabling steps).