Remove deprecated text property from Word

[jo-elimu]

https://github.com/elimu-ai/webapp/blob/main/src/main/java/ai/elimu/model/content/Word.java#L21

When creating a new Word, a content creator will no longer type the text of a Word. Instead, she will select the Word's letter-sound correspondences. And the Word text will then be replaced by concatenating the letters in the list of letter-sound correspondences.

• Remove from JPA entity
• Add toString() method to entity
• Remove from database table (add DB migration script)
• Update controller classes
• Update frontend files
• Update CSS files