Sponsor city attribute

https://osraav.atlassian.net/browse/OSRA-206

*The original plan was to create a Place model to store cities and countries. During implementation, this approach turned out to be awkward and to involve much redundancy without justifying these shortcomings.*

- add attribute .city to Sponsor
- add City drop-down box to Sponsor form, populated with existing Sponsor.city values
- add New City Name text box to allow entry of new cities
- implement Sponsor#set_city to add new city names

`pluck`ing city names from the Sponsors table to populate the select box should be sufficiently performant, but there are alternatives we can consider in the future.

OSRA-206 #Code-Review
is it worth making `**Add New**` a constant?