[timepoint] Add Default Site to Dropdown

[victori444]

Brief summary of changes

In the create timepoint module, adds a candidate's default site (if they belong to many sites) to the 'Site' dropdown

Testing instructions (if applicable)

1. Create a new timepoint for any candidate
2. Under the 'Site' option, verify that the candidate's default site is already populated
3. Create the timepoint and ensure all functions correctly

CCNA Override