Add 'locale' as an attribute on the User Resource

[danielbachhuber]

Now that users can pick their own locales, this data should be exposed through the REST API.

[ocean90]

First pass in 38528.patch​.

[joehoyle]

New patch with support for updating the user's local. Also, I changed to use an ENUM of available languages, so this means there's no need for sanitization as only values in get_available_languages() are then allowed.

Added a couple more tests too.

[rmccue]

In 39090:

REST API: Add locale to user resource.

Props ocean90, joehoyle.
Fixes #38528.

[ocean90]

get_available_languages() doesn't return en_US, see 38528.3.diff​ for a failing test. I'm currently not sure if an empty string (to force using get_locale() in get_user_locale()) should be supported as well.

cc: @swissspidy

[joehoyle]

In 39096:

REST API: Add en_US to the locales enum.

Props ocean90.
Fixes #38528.