ENH: Localisation manager filter.

[mfendeksilverstripe]

Description

Enables filtering capability in the localisation manager. This filter wasn't present because pre CMS 5 such feature was not supported. With CMS 5 it's possible so we should enable it.

Manual testing steps

• Navigate to the localisation manager on any localised model and confirm that the filter is working from scaffolded fields

Issues

#924

Pull request checklist

• The target branch is correct
  • See picking the right version
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• CI is green

[tractorcow]

Looks good to me. Generally the locales list isn't large enough that a search is required, but it's no harm to have it working. :)

[mfendeksilverstripe]

FYI: Squashed commits.