Allow overriding all words and strings via settings.php, or show overridable status in the interface

Problem/Motivation

In settings.php, there's a section about String overrides, which is great. For example:

$settings['locale_custom_strings_en'][''] = [
  'Home' => 'Frontpage',
  '@count min' => '@count minutes',
];

But some strings can't be overridden, such as "forum". Ideally, all strings and words in the interface should be overridable.

Currently, there isn't an easy way to check if a word or a string is overridable. Until all strings and words can be overridden, it would be awesome if it was possible to somehow easier discern between overridable strings, and those not, directly in the interface.

Steps to reproduce

Try to override some words or strings such as "Forum", and see that nothing happens.

Proposed resolution

1. Allow all words and strings to be overridden.
2. Until all strings and words can be overridden, add a feature which shows override status for words and strings directly in the interface, possibly using Devel?

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet