Make `ModelAdmin` abstract and remove useless `allowed_action` declaration

[GuySartorelli]

There are a few small bits about ModelAdmin which aren't ideal. There are a few other issues as well, but these two are so small they weren't worth being separate from one-another.

Related

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

Acceptance criteria

• Remove SearchForm from allowed_actions - this is handled by gridfield, not modeladmin
• Make ModelAdmin abstract