Add the `wifi5.outdoor_chanlist` site configuration that
allows specifying an outdoor channel range that can be
switched to for regulatory compliance.

Upon enabling the outdoor option the device will
 - configure the `outdoor_chanlist` on all 5 GHz radios
 - which may enable DFS/TPC, based on the regulatory domain
 - disable ibss/mesh on the 5 GHz radio, as DFS *will*
   break mesh connections
 - allow for htmode reconfiguration on 5 GHz radios

The outdoor option can be toggled from
 - Advanced Settings
   - W-LAN
     - Outdoor Installation

The `preserve_channel` flag overrules the outdoor channel
selection.

- 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

simple-uci will now delete an option when nil is passed.