- Replace string concat for i18n key with an explicit list to make the code
  compatible with i18n-scan.pl
- Improve fallback string

- CGI script and index.html are moved from gluon-web to
  gluon-config-mode-core, the script is renamed to 'config'
- gluon-web and gluon-web-model base views and i18n files are symlinked
  into the new path
- gluon-web-theme is renamed to gluon-config-mode-theme and installs
  directly into the new path
- all gluon-web-* models, controllers and views are moved into the new
  path

The 'auto' attribute still allows enabling the interface using ifup, which
is not intended when wired mesh is disabled.

Fixes #1269