Fixed capitalization of API and CTA in theme setting labels

no issue

- added `{{humanize-setting-key}}` helper that uses the base `humanize` output but then forces capitalization for known acronyms
- updated `<CustomThemeSettings::Select>` to use the new helper

ghost/admin/app/components/custom-theme-settings/select.hbs

@@ -1,7 +1,7 @@
 <div class="gh-stack-item {{if (eq @index 0) "gh-setting-first"}}">
     <div class="flex-grow-1">
         <label class="gh-setting-title gh-theme-setting-title" for={{this.selectId}}>
-            {{humanize @setting.key}}
+            {{humanize-setting-key @setting.key}}
         </label>
 
         <select class="ember-select" name={{this.selectName}} id={{this.selectId}} {{on "change" this.setSelection}}>

ghost/admin/app/helpers/humanize-setting-key.js

@@ -0,0 +1,16 @@
+import {helper} from '@ember/component/helper';
+import {humanize} from 'ember-cli-string-helpers/helpers/humanize';
+
+export function humanizeSettingKey([key]) {
+    let humanized = humanize([key]);
+
+    const allCaps = ['API', 'CTA'];
+    allCaps.forEach((str) => {
+        const regex = new RegExp(`(^| )(${str})( |$)`, 'gi');
+        humanized = humanized.replace(regex, `$1${str}$3`);
+    });
+
+    return humanized;
+}
+
+export default helper(humanizeSettingKey);