fix: Block inline images in message signatures (#13772)

# Pull Request Template

## Description

This PR includes, block inline images in message signatures and prevent
auto signature insertion when editor is disabled.

- Strip inline base64 images from signature on save and show warning
message
- Add `INLINE_IMAGE_WARNING` translation key for signature inline image
removal notification
- Add disabled check to `addSignature()` to prevent signature insertion
when editor is disabled
- Add `isEditorDisabled` checks to signature toggle logic in
`toggleSignatureForDraft()`, `replaceText()`, and `clearMessage()`
- Remove unused `replaceText` from the codebase, which belongs to old
`textarea` editor

Fixes
https://linear.app/chatwoot/issue/CW-6588/the-browser-hangs-when-the-message-signature-contains-inline-image

## Type of change

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

## How Has This Been Tested?

### Loom video
https://www.loom.com/share/fb556b46a12a4308a737eed732d5ed73


## 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

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/javascript/dashboard/i18n/locale/en/settings.json

@@ -68,7 +68,8 @@
         "API_SUCCESS": "Signature saved successfully",
         "IMAGE_UPLOAD_ERROR": "Couldn't upload image! Try again",
         "IMAGE_UPLOAD_SUCCESS": "Image added successfully. Please click on save to save the signature",
-        "IMAGE_UPLOAD_SIZE_ERROR": "Image size should be less than {size}MB"
+        "IMAGE_UPLOAD_SIZE_ERROR": "Image size should be less than {size}MB",
+        "INLINE_IMAGE_WARNING": "Pasted inline images were removed. Please use the image upload button to add images to your signature."
       },
       "MESSAGE_SIGNATURE": {
         "LABEL": "Message Signature",