Allow for translatable tab names in `ModelAdmin`

[GuySartorelli]

Currently tab titles in ModelAdmins are either hardcoded or come from calling i18n_plural_name() on a singleton of the model.
This means if you want a specific localisable tab name you have to hack it in yourself.

Related

• SPIKE Analyse how LeftAndMain and CMSMain are put together and how they could work better silverstripe-cms#2933

Acceptance criteria

• Provide a way to give i18n replacements for tab titles without having to override getManagedModelTabs() or getManagedModels()

[GuySartorelli]

This can probably be done in a minor release, but would be easier if we don't have to worry about keeping things perfectly BC.