Introduces the ability to enable webpack DLL support by passing
the flag WEBPACK_VENDOR_DLL=true while running webpack or
webpack-dev-server.

This removes the usage of IS_GITLAB_EE
and prefer to use FOSS_ONLY that has
the following states:
1. nil/empty string: use CE or EE depending on sources
1. false: use CE or EE depending on sources
2. true: use CE always

Currently:
1. If `IS_GITLAB_EE=` is used, it fails to generate Webpack
   as it cannot be parsed as JSON,
2. If `IS_GITLAB_EE=1` is used, it fails on GitLab CE,
3. If `IS_GITLAB_EE=0` it does work correctly, and disables EE,
4. If `IS_GITLAB_EE=1` is used on GitLab EE it effectively required
   to run on EE changes.

This changes to:

Make `IS_GITLAB_EE=` to donate the intent to disable or enable EE
sources, instead of requiring it, and makes the check consitent
in behaviour.

(cherry picked from commit 64e9cf00)

(cherry picked from commit baa671a3)