feat: Add plain-text editor for non-rich content channels (#13058)

# Pull Request Template

## Description

This PR restores the plain text editor for all channels except Website,
Email, and API.

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/shared/components/ResizableTextArea.vue

@@ -111,10 +111,15 @@ export default {
     // watcher, this means that if the value is true, the signature
     // is supposed to be added, else we remove it.
     toggleSignatureInEditor(signatureEnabled) {
-      const valueWithSignature = signatureEnabled
+      let valueWithSignature = signatureEnabled
         ? appendSignature(this.modelValue, this.cleanedSignature)
         : removeSignature(this.modelValue, this.cleanedSignature);
 
+      // Clean up whitespace when removing signature from empty body
+      if (!signatureEnabled && !valueWithSignature.trim()) {
+        valueWithSignature = '';
+      }
+
       this.$emit('update:modelValue', valueWithSignature);
       this.$emit('input', valueWithSignature);