Language dropdown

Change the language input for a user from a text box to a combobox dropdown in response to user feedback at https://dimagi.uservoice.com/forums/194748-user-management/suggestions/3918046-drop-down-box-for-languages-when-adding-users.

If there are projects, then the list will have only languages that are used in these projects. Otherwise the list will be the entire list of languages that is used when adding a language to an application.

This pull request depends on features implemented in a pull request submitted to the langcode submodule (https://github.com/dimagi/langcodes/pull/1).