Clarify when to use modelTab vs modelClass in ModelAdmin - and use them correctly.

[GuySartorelli]

modelTab and modelClass are used interchangably right now, and it's not always clear which should be used at a given time. This can cause problems.

Related

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

Notes

• This is intentionally very vague - it could be a matter of documenting best practice and following it, or it could be a matter of changing the API for ModelAdmin so it is more directly intuitive.

Acceptance criteria

• Make clearer rules about when to use modelTab and when to use modelClass
• Follow those rules in all relevant places in supported modules