added :patrons_can_renew to app configs
updated ability.rb with patron renewal option
updated reservation show page based on renewal ability
made items with zero-length max renewal ineligible for renewal
updated schema
Renamed configuration parameter to enable_renewals
broadened restrictions to all roles except superuse
update app config model with new parameter name
update schema again
Merge pull request #921 from YaleSTC/916_renew_button

[v3.4] 916 renew button