Salesforce scheduling issues

[axelstudios]

What's this PR do?

• Fixes an issue where attempting to set a Salesforce sync time with hour = 0 or minute = 0 would (in most cases) not actually take effect
• Fixes an issue where each time a Salesforce config is saved with a cron schedule that hasn't previously been used it orphans the old schedule, resulting in tons of unused records in the crontabschedule table:
  
• Improves the UI to use restricted numeric inputs for hours/minutes, which fixes mixed data types when saving a Salesforce config:
  

How should this be manually tested?

1. Modify the hour/minute of a Salesforce scheduled update and:
   1. Check that the PUT request uses integers for update_at_hour and update_at_minute
   2. Check that setting the hour or minute to 0 correctly updates the crontabschedule table
   3. Check that the crontabschedule table has no entries that aren't referenced by the periodictask table