- add base.html template with head, header, footer optional customizations
- refactor all top-level templates to extend base.html, except frame_insert.html
- localization: add placeholder support for jinja2 localization extension, '{% trans %}' and _('') tags, placeholder null localization
- refactor new query UI to support localization
- update some text to match localized versions used in ukwa-pywb, update test