Support system.* fields and system_* fields in backend

In order to fix https://github.com/fabric8-services/fabric8-wit/issues/2177 we need to support system_ fields along with system. fields.

This issue involves

[x] Renaming of fields in source code https://github.com/fabric8-services/fabric8-wit/pull/2330 (merged)
[x] Adding helper methods to convert the incoming system.* field to system_* field and updating /workitem request/response to include both system.* and system_* fields https://github.com/fabric8-services/fabric8-wit/pull/2333
[x] Updating /workitemtype/list and /workitemtype/show response to return both system.* fields and system_* fields. https://github.com/fabric8-services/fabric8-wit/pull/2335
[x] Updating /workitem/event/list to return both system.* fields and system._* field name. https://github.com/fabric8-services/fabric8-wit/pull/2337
[x] Database migration script to update all workitems, workitem_revisions and workitemtypes to use system_* field names https://github.com/fabric8-services/fabric8-wit/pull/2340
[x] Database migration script to fix the TSV vector and triggers which depend on system.* fields https://github.com/fabric8-services/fabric8-wit/pull/2342

fabric8-services / fabric8-wit