Move `g3w-admin/core/static` into `g3w-admin/static`

[Raruto]

Related to: https://github.com/g3w-suite/g3w-suite-docker/pull/62

As already happens for the templates folder, we can also set a folder where to place all core static files

(see: https://docs.djangoproject.com/en/2.2/ref/settings/#std:setting-STATICFILES_DIRS)

For example here's how it might work:

Before (v3.4)

https://github.com/g3w-suite/g3w-admin/blob/12024ef814cfe1c2bd56f097936521d6988e6c67/g3w-admin/base/settings/base.py#L225-L228

After

# Static files (CSS, JavaScript, Images)
# https://docs.djangoproject.com/en/1.9/howto/static-files/

STATIC_URL = '/static/'
STATICFILES_DIRS = [os.path.join(BASE_DIR, '../static')]

NB before proceeding, the various build scripts (eg. bower components) must also be checked and updated accordingly.