This will hide the outdoor mode setting on compatible devices in case
the defined channels should be preserved.

Otherwise a user might be under the impression their device is compliant
with outdoor operation when in reality it still uses prohibited
channels.

When disabling outdoor mode, 5GHz mesh interfaces should be enabled by
default again (unless disabled in site.conf).

Instead of relying on the existence of interface sections only (which is
still used for the case of interface types that are not configured at
all in the site config), add a proper dependency.

This fixes section visiblity with enabled outdoor mode after the following
fix, and gives the user immediate feedback (allowing to disable outdoor
mode and enable meshing in a single step).

- Return early
- Rename to has_active_interfaces(), as it returns a boolean

200-wireless will add or remove the mesh network sections of
/etc/config/network. Commit this file, so the modified setting doesn't
get lost on reboot.

Fixes: #2048

Separate wireless-related helper methods from the util module to a
new wireless module. This keeps them separated, as the amount of
wireless helpers increased in the past, justifying a separate module.

Fixes #1809

Fixes: bf552491 ("gluon-core: add outdoor support for 5 ghz radios")

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.