Contributions for last deriverable Carmen Rendueles Martínez

[carmen279]

For the last deliverable, I contributted to the proyect by solving the following issues:
Fetch friends from pod in background:
#196
Calculate and update distances in background:
#195
Notifications for new friend or close friend:
#194
Notifications refactor and bug:
#203
Bug loading close friends to main screen:
#204
Bug getting the location (needed to check permissions):
#197
View for adding pictures to locations:
#213
Message offering to share the app when no friends:
#214
Testing app with newly created and empty pod:
#215
Testing notifications in real conditions (this gave new problems):
#216
Change to expo-background-fetch for background tasks:
#247
Solved several bugs with @uo270927:
#272
Built the apk of the mobile application:
#273
Background tasks clean up:
#277
Solved several bugs found when dealing with the apk:
#276
Fixed notification bugs:
#275