Allow set up of new locales in projects with both, mono- and bilingual files

[bugzilla-to-github]

This issue was created automatically by a script.

Bug 1728082

Bug Reporter: @mathjazz

When new locales are added to projects, Pontoon creates relevant files and folders behind the scenes if they don't exist in the repository.

If the project contains a mixture of monolingual (asymmetric) and bilingual files (as is the case in Firefox Accounts), bilingual files may not get created.

Let's fix that.

[mathjazz]

The underlining issue is that we only check the first project resource and then assume all other resources in the project are of the same type (monolingual or bilingual).

[mathjazz]

Lowering priority since we don't have such projects.