Centralize `support_url.txt` and fix redirect

Currently, we have dedicated support_url.txt files for each locale:

➜ g grep ios.forums -- fastlane
fastlane/metadata/ar-SA/support_url.txt:https://ios.forums.wordpress.org
fastlane/metadata/da/support_url.txt:https://ios.forums.wordpress.org
fastlane/metadata/de-DE/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/en-AU/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/en-CA/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/en-GB/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/en-US/support_url.txt:https://ios.forums.wordpress.org
fastlane/metadata/es-ES/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/es-MX/support_url.txt:https://ios.forums.wordpress.org
fastlane/metadata/fr-FR/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/id/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/it/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/ja/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/ko/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/nl-NL/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/no/support_url.txt:https://ios.forums.wordpress.org
fastlane/metadata/pt-BR/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/pt-PT/support_url.txt:https://ios.forums.wordpress.org
fastlane/metadata/ru/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/sv/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/th/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/tr/support_url.txt:https://ios.forums.wordpress.org
fastlane/metadata/zh-Hans/support_url.txt:http://ios.forums.wordpress.org/
fastlane/metadata/zh-Hant/support_url.txt:http://ios.forums.wordpress.org/

That's not a big deal, but given the value is the same, we might as well use a single file in fastlane/metadata/default.

What's more of a concern, is that the URL redirects to https://wordpress.org/mobile/. This should be fixed, whether by pointing to the updated forums URL ( https://wordpress.org/support/forum/mobile/ ) or update it to https://wordpress.org/mobile/ to avoid a redirect.

Inspect the locale-specific fastlane/metadata/*/support_url.txt files and the proposed fastlane/metadata/default/support_url.txt location. Consolidate the identical support URL there, select the updated forums or mobile URL described in the issue, and verify the resulting URL no longer redirects.