Database model, null to belongs_to

* Can assign NULL to belongs_to relation if it's defined as null
Validation: get_model

* Added validation for get_model function
Null to belongs_to

* Added proper null belongs_to handling
FS caching

* Added cfg for system config caching
* Added cfg for resource caching
Response, default format

* Default http response is html
System status, session

* Dump bar dumps session only if available
User model, com

* Dropped deprecated user attrs com_email, com_sms
Contact model, comm

* Added spam attr to user contact model
* Dropped deprecated visible attr
Template aliases

* Added template aliases for table, tbody, thead, th, tr, td
* Fixed template alias for head
[fix] Default config

* Fixed default cache config
Imprecise datetimes

* Added helper to format imprecise datetimes (via locales)
Image input 3

* Tweaked UI for image input
Forms, Image widget 3

* Image widget can download remote images
Forms, Image input 3

* Added missing colon to image input label