Common forum settings

As an Administrator i want to have forum settings inside JCommune, without using Poulpe.

Acceptance criteria

1. Options are available in administration mode in existing "Administration" popup
2. Following options should be added:

   Label EN	Label RU	Label UA	Value range
   URL address	URL адрес	URL адреса	0..255 symbols
   Session timeout	Таймаут сессии	Таймаут сесії	0..1440
   Avatar max size	Максимальный размер аватара	Максимальний розмір аватару	0..1048576 bytes (1 Mb)
   Email notifications	Email уведомления	Email сповіщення	Checked/Unchecked

Found bugs

Description	Status
Avatar max size option's placeholder is not correct in Ukrainian
The error message for Avatar max size option had better contain 'bytes' unit
Email notifications checkbox doesn't show saved 'Unchecked' value
Impossible to move through all new options by pressing Tab, Shift + Tab