Replace !placeholder with @placeholder in format_string() for non-URLs

Follow-up to #2506445: Replace !placeholder with @placeholder in t() and format_string() for non-URLs in tests

Problem/Motivation

First step of #2506427: [meta] !placeholder causes strings to be escaped and makes the sanitization API harder to understand.

Proposed resolution

• Replace !placeholder with @placeholder, in format_string()/SafeMarkup::format() usage only, excluding those used in URLs or non-html formats.

Bash command to use:

egrep -r '\format_string\(.*\!([a-zA-Z])+' * | grep -v 'href="!' | grep Test.php | grep -v "vendor" | grep -v ".js" | grep -v .css | grep -v "~" 

Remaining tasks

TBD

User interface changes

Ideally, fewer completely unexpected double-escaping bugs.

API changes

None.