As a user, I want to be able to view and leave comments about dishes and restaurants.

Description:
Patat does not allow any user feedback regarding the data provided.

Checklist view comments
- [x] database changes
- [x] everybody can view comments
- [x] order comments by date; most recent comment first
- [x] comments go along with username, date
- [x] comments should be limited to 5000 characters
- [x] display comments in restaurant view below restaurand/dish but collapsed by default 

Checklist add comments
- [x] comments can only be written if user (which is a customer) is logged in
- [x] if user is logged in, there is a textbox below existing comments to a restaurant/dish to create a comment
- [x] implement a new control that supports character count and use it for creating comments
- [x] if a dish is deleted, its comments are deleted as well
- [x] make view of comments prettier (e.g. make it visible for the user that 'Toggle comments' can be clicked on)
- [x] a user can add muliple comments to the same dish
- [x] comment description is too long
- [x] reload view, after comment has added to dish/restaurant
- [ ] validate comment description before save

@anna-averianova Almost done, except from validation of description, which I need help with. Can we take a look at that tomorrow? The rest is ready for review.