Migration to add scheduled_start_time to race_entries table

[moveson]

Database migration to add the scheduled_start_times column to the race_entries table.

Addresses a portion of #64

[moveson]

Migration is up in production:

17250-MOveson:rattlesnake_ramble markoveson$ heroku run rails db:migrate:status
Running rails db:migrate:status on ⬢ rattlesnake-ramble... up, run.8607 (Hobby)

database: dccm2q6ij6ne31

 Status   Migration ID    Migration Name
--------------------------------------------------
   (1.2ms)  SELECT "schema_migrations"."version" FROM "schema_migrations" ORDER BY "schema_migrations"."version" ASC
   up     20170730042048  Create racers
   up     20170730165309  Create races
   up     20170730165811  Create race editions
   up     20170730170044  Create products
   up     20170730170147  Create race entries
   up     20170815182454  Create product images
   up     20170906150107  Add slug to race editions
   up     20170906154037  Add slug to races
   up     20170906210843  Add bib number to race entries
   up     20190913124555  Add offsets to race editions
   up     20200904043442  Add scheduled start time to race entries