Location field submission should depend on modality

[caitbsav]

Describe the bug
When proposing a workshop, you shouldn't need to enter text into both Virtual location and In person location fields unless the modality is hybrid.

To Reproduce
Steps to reproduce the behavior:

1. Go to (https://test.workshops.library.gwu.edu/workshops/new)
2. Propose a virtual-only presentation modality
3. Enter in a virtual location but not an in person location
4. Hit Submit
5. Receive a "Please fill out this field." error when attempting to submit.

Expected behavior
I would expect to receive a location field that's conditional based on what I selected under modality:

Select Virtual, receive Virtual Location field
Select In-Person, receive In-Person Location field
Select Hybrid, receive both Virtual Location and In-Person Location fields

if conditional appearance isn't doable, I would just make these optional, not required

Screenshots
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