Don't render TOC in emailed note notifications

What does this MR do and why?

Fixes TOC (Table of Contents) tags should not be rend... (#581849).

Note email templates re-render Markdown from scratch using EmailPipeline, which (unlike NotePipeline) doesn't set no_header_anchors. This caused [[_TOC_]]-style tags to be rendered in the email notification, even though the note itself wouldn't expand them.

Fix by passing no_header_anchors: true in the note email partial. We don't want to modify EmailPipeline itself since it does actually need to render TOCs sometimes (e.g. for issue/MR descriptions in notifs).

How to set up and validate locally

1. Check out the branch.
2. Create a note on an issue or MR that looks like this:

   [[_TOC_]]
   
   # Hello, world!
   
   ## Olá, mundo!
   
   @someuser

   You should @mention a user who isn't the one you're logged in as, to trigger an email notification.
3. Visit /rails/letter_opener. You should see something like this:
   
   Why is it blocked? I dunno, our Letter Opener configuration seems to be a bit wonky. You can middle-click the email at left to open it in a new tab. It should look like this:
   
   Note it says _TOC_ as a link. This is correct — it matches the actual render on the site:
   
   An incorrect result would be an actual table of contents appearing in the email notif, which is the current behaviour.

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