Translate custom block type templates

As [discussed a while ago](https://github.com/concrete5/concrete5/pull/1455#issuecomment-27805300), let's add the `BlockTypeCustomTemplateName` and `BlockTypeComposerTemplateName` translation contexts to translate the names of the custom templates of the block types.

See also https://github.com/mlocati/concrete5-localizer/commit/314ad9ae690077bc64689a01ed14f7dc5dd35e07