Allow nested locations

[JacobEvelyn]

I find myself adding activities with descriptions like Grabbed lunch with **Julie Andrews** in Cambridge (_Boston_) because I want the description to count as being in Boston when it technically isn't. If friends allowed me to specify that Cambridge is within Boston it could keep this specificity (as opposed to just being a nickname, as described in #242).

I think the big thing to figure out would be how to store this data in the file. Maybe a nested list structure, like:

### Locations:
- Boston
    - Cambridge
    - Somerville
- Europe
    - France
        - Paris
    - London
- New York City

Would love feedback on this!