enable normal mode by default in Custom-mode buffers

Originally reported by: **Wouter Bolsterlee (Bitbucket: [wbolster](https://bitbucket.org/wbolster), GitHub: [wbolster](https://github.com/wbolster))**

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Is there a reason Custom-mode buffers are in Emacs state by default? There is some special handling of buttons and so on inside evil, so it seems to me there is no reason to treat Custom-mode (note the capital) as special...

```lisp
(evil-set-initial-state 'Custom-mode 'normal)
```

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- Bitbucket: https://bitbucket.org/lyro/evil/issue/640

Start by tracing how Evil assigns initial states to major modes, focusing on the existing handling of Custom-mode and the `evil-set-initial-state` entry point shown in the issue. Done means Custom-mode buffers enter normal mode by default without requiring the manual configuration form.

Written by the indexing model from the issue text.