TSLint is only run on app/ directory

Created by: felixfbecker

Currently TSLint only checks the app/ directory in CI. Changing it to lint all folders reveals the following errors:

ERROR: /Users/felix/src/github.com/sourcegraph/browser-extensions/chrome/extension/background.tsx[114, 1]: Promises must be handled appropriately
ERROR: /Users/felix/src/github.com/sourcegraph/browser-extensions/chrome/extension/background.tsx[268, 5]: Promises must be handled appropriately
ERROR: /Users/felix/src/github.com/sourcegraph/browser-extensions/chrome/extension/background.tsx[276, 5]: Promises must be handled appropriately
ERROR: /Users/felix/src/github.com/sourcegraph/browser-extensions/chrome/extension/background.tsx[321, 5]: Promises must be handled appropriately
ERROR: /Users/felix/src/github.com/sourcegraph/browser-extensions/extension/safari/StorageArea.ts[65, 40]: Explicit type parameter needs to be provided to the constructor
ERROR: /Users/felix/src/github.com/sourcegraph/browser-extensions/extension/storage.ts[91, 40]: Avoid referencing unbound methods which may cause unintentional scoping of 'this'.
ERROR: /Users/felix/src/github.com/sourcegraph/browser-extensions/extension/storage.ts[91, 50]: Avoid referencing unbound methods which may cause unintentional scoping of 'this'.
ERROR: /Users/felix/src/github.com/sourcegraph/browser-extensions/scripts/tasks.ts[12, 14]: Avoid referencing unbound methods which may cause unintentional scoping of 'this'.
ERROR: /Users/felix/src/github.com/sourcegraph/browser-extensions/scripts/tasks.ts[13, 14]: Avoid referencing unbound methods which may cause unintentional scoping of 'this'.

These seems to be real problems, e.g. Promises without error handlers or that are not awaited, or unbinding methods.