Adding condition for skipping symlinks when installing extensions via the extension manager

When installing extensions that have symlinks via the extension
manager, they will fail with a message regarding an invalid zip file.
This can be tested with
https://github.com/MiguelCastillo/Brackets-SymbolicLinks
Merge branch 'master' into MiguelCastillo-master
Adds a test for files with symlinks
Merge branch 'MiguelCastillo-master' (PR #5919)