Support system.* fields and system_* fields in backend

[jarifibrahim]

In order to fix #2177 we need to support system_* fields along with system.* fields.

This issue involves

• Renaming of fields in source code Rename system.* to system_* in a some files #2330 (merged)
• Adding helper methods to convert the incoming system.* field to system_* field and updating /workitem request/response to include both system.* and system_* fields Add temporary code for field rename #2333
• Updating /workitemtype/list and /workitemtype/show response to return both system.* fields and system_* fields. Update /workitemtype response for field rename #2335
• Updating /workitem/event/list to return both system.* fields and system._* field name. Add onField to the /workitem/event response #2337
• Database migration script to update all workitems, workitem_revisions and workitemtypes to use system_* field names Add migrations for field name rename #2340
• Database migration script to fix the TSV vector and triggers which depend on system.* fields Enable skipped test for field rename and update TSV trigger #2342