Add more options to the DeepL settings (Fixes #831)

[alexkiro]

• Add the ability to configure glossaries by (source, target) language pairs.
• Add the ability to configure the timeout instead of having it hardcoded to 30.
• Add tests for the new glossary options.
• Update docs for the new options.

Fixes #831

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I've also extracted constructing the parameters into a separate method. This way if anyone else needs even more advanced customizations of the parameters it's easier to extend:

from wagtail_localize.machine_translators.deepl import DeepLTranslator


class CustomDeepLTranslator(DeepLTranslator):
    def get_parameters(self, source_locale, target_locale, strings):
        parameters = super().get_parameters(source_locale, target_locale, strings)
        parameters["another_option"] = "My custom value"
        return parameters