[#804] Use default_locale when lang not set in the request

[krakan]

Before a language is selected in a Pywb session it's expected that the default_locale from the config-file would be used. This works most of the time but not for eg. formatting dates on the query.html page where instead the en locale is used.

Description

Let the FrontEndApp __init__ method set a default_locale attribute as defined in the config file and use that as the default for incoming requests.

Motivation and Context

See issue #804.

Screenshots (if appropriate):

See issue #804.

Types of changes

• Replay fix (fixes a replay specific issue)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added or updated tests to cover my changes.
• All new and existing tests passed.

[tw4l]

Thanks for this @krakan ! :)