Added support for text custom theme settings

refs https://github.com/TryGhost/Team/issues/1109

- added basic `<CustomThemeSettings::Text>`  component that is displayed for any custom theme settings with the type `'text'`

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

@@ -0,0 +1,17 @@
+<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.inputId}}>
+            {{humanize-setting-key @setting.key}}
+        </label>
+
+        <input
+            type="text"
+            class="gh-input"
+            value={{@setting.value}}
+            id={{this.inputId}}
+            name={{this.inputName}}
+            {{on "input" this.updateValue}}
+            {{on "blur" this.triggerOnChange}}
+        />
+    </div>
+</div>

ghost/admin/app/components/custom-theme-settings/text.js

@@ -0,0 +1,19 @@
+import Component from '@glimmer/component';
+import {action} from '@ember/object';
+import {camelize} from '@ember/string';
+import {guidFor} from '@ember/object/internals';
+
+export default class CustomThemeSettingsTextComponent extends Component {
+    inputId = `input-${guidFor(this)}`;
+    inputName = camelize(this.args.setting.key);
+
+    @action
+    updateValue(event) {
+        this.args.setting.set('value', event.target.value);
+    }
+
+    @action
+    triggerOnChange() {
+        this.args.onChange?.();
+    }
+}

ghost/admin/app/components/settings/design/theme-settings-form.hbs

@@ -10,6 +10,9 @@
             {{#if (eq setting.type "color")}}
                 <CustomThemeSettings::Color @setting={{setting}} @index={{index}} @onChange={{@updatePreview}} />
             {{/if}}
+            {{#if (eq setting.type "text")}}
+                <CustomThemeSettings::Text @setting={{setting}} @index={{index}} @onChange={{@updatePreview}} />
+            {{/if}}
         {{/each}}
     </form>
 </div>