FEATURE: Allow string theme settings to display with multiple lines

To use, add `textarea: true` to the theme settings.yml. For example:

```
my_setting:
  default: "some string"
  textarea: true
```

spec/components/theme_settings_manager_spec.rb

@@ -128,6 +128,12 @@ describe ThemeSettingsManager do
 
       expect { string_setting.value = ("a" * 21) }.to raise_error(Discourse::InvalidParameters)
     end
+
+    it "can be a textarea" do
+      string_setting = find_by_name(:string_setting_02)
+      expect(find_by_name(:string_setting_02).textarea).to eq(false)
+      expect(find_by_name(:string_setting_03).textarea).to eq(true)
+    end
   end
 
   context "List" do

spec/fixtures/theme_settings/valid_settings.yaml

@@ -10,6 +10,10 @@ string_setting_02:
   min: 2
   max: 20
 
+string_setting_03:
+  default: "string value"
+  textarea: true
+
 integer_setting: 51
 
 integer_setting_02: