Make language selector being available in all templates

[magicznyleszek]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a description of your work suitable for publishing on our forum
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Notes

I simply moved the existing <form> to upper scope, so it is being rendered in all the pages.

Related issues

Part of #3804

[noliveleger]

@magicznyleszek, does the inheritance of base.html make it appear in password reset template too?

[magicznyleszek]

@magicznyleszek, does the inheritance of base.html make it appear in password reset template too?

Yes, now it will appear everywhere. I assumed we want the option to change the language to be everpresent