feat: add reactions to question & answer

[hgaol]

close #883 .

done backend:

• used meta table to save reactions.
• created 2 APIs. one is to retrieve meta for an object, the other is to add or update.

frontend:

• added an Reaction component.
• to make it easy to determine if it's need to show the Add a comment button, place it in the Comment component.

todo

• [x] add i18n for reactions.