Refactor common entry point for standard filters

Create filter_standard_get() to build a standard filter.
Remove filter_load() as a multipurspose filter function. Standard
filters require a varying set of parameters, which add noise for the
case of retrieving a raw filter by id.

At this point, these are the functions to retrieve filters:

• filter_get(): get a filter from database by id
• filter_temporary_get(): get a temporary filter, by it's id. These
  filters are stored as session data at user's scope.
• filter_standard_get(): build a filter for the standard filters
  predefined in the application.