Remove external sources

[friederbluemle]

This is a work-in-progress PR (currently does not build).

It removes the copies of all external source files (2x react-native, and react-native-electrode-bridge) from the repository. The goal is to acquire and integrate with external dependencies through a package and/or dependency manager (like other projects).

⚠️ do-not-merge

[friederbluemle]

Nice. After #67 is merged, this PR is down from

2105 files changed, 4547 insertions(+), 236247 deletions(-)

to

1055 files changed, 4547 insertions(+), 118129 deletions(-)