Orphan priority

**This PR is branched off #64 which should be merged first.**

JIRA story https://osraav.atlassian.net/browse/OSRA-72
- add `priority` attribute to Orphan, acceptable values 'Normal' & 'High'
- add `high_priority` scope to Orphan (currently not used anywhere)
- set default priority to 'Normal'
- add priority to orphan index view with CSS highlight when 'High'
- add priority to orphan edit view
- ensure selectors in orphan edit view do not have choice of blank value


## JIRA
OSRA-72 #Code-Review
OSRA-73 #Code-Review
OSRA-74 #Code-Review
OSRA-75 #Code-Review
OSRA-76 #Code-Review