handle mod_article_teaser and mod_article_plain selection

[fritzmg]

In Contao 3 you were able to select a variety of custom template for articles, including mod_article_plain and mod_article_teaser which were removed in Contao 4.

However, this will lead to an

Could not find template "mod_article_plain"

error after upgrading to Contao 4 if these template were previously set as a customTpl manually (for whatever reason) and no such template is present as a custom template.

Thus may be Version400Update should check whether or not a custom template with these names are present and if not, tl_module.customTpl should be emptied for these records.

[ghost]

The issue has been moved to contao/contao#21.