Update the endpoint handling

Related to #21 (closed).

We need to append the static endpoint with the district's name as a lower-case string. See #21 (closed) for full description of the overarching reason.

We should do this portion as if there's a dummy variable that we'll use. So:

1. Create a dummy string variable that contains a district (e.g., var district_name = "eastern")
2. Update the static endpoint string at line 59 in frontend/src/PhotoUploadForm.js to include the trailing slash (to delimit it from the remaining info)
3. Add the new variable to the end of that endpoint to concatenate the strings into one endpoint URL.