forms: Remove (some of the) magic from field config
forms: Any field can have sub fields

Not just the SelectionField. This will make room for the InlineFormField
forms: Allow for more flexible advanced search integration
lists: Remove (some of the) magic from properties
search: Selection lists always allow for multiple selections

Furthermore, revamp the __cdata system to better support multiple selection
fields. Drop the `.._id` fields and use a coalesce of the ID value of the
entry value or the entry value itself. For JSON backed fields, strip some
JSON characters to result in a comma-separated list.

The search engine now uses MySQL FIND_IN_SET to analyze a search selection
in advanced searches. Also, interaction with the MySQL full text search
backend is fully supported.
search: Fix _N() usage for advanced search
search: Drop CDATA table on upgrade for new format
Merge pull request #1214 from greezybacon/oops/forms

forms: Several minor fixes for properties and search integration

Reviewed-By: Peter Rotich <peter@osticket.com>