upgrade moment dependency

[shiftkey]

I just got a notification about CVE-2017-18214 which was raised against the moment project: moment/moment#4163

We use moment in a few places, and the ReDos vulnerability is an interesting read, but until I hear more I'm going to leave this open for someone to have a go at:

• upgrading the dependency in app/package.json to 2.19.3 or later

desktop/app/package.json

Line 39 in 6eb4064

"moment": "^2.17.1",

• run yarn from the root to update the lock file

• commit both of these changes

• test out the app to ensure it still renders relative dates correctly

Places to check as part of verifying the upgrade:

• the history view - each commit shows a relative time in the description up until some limit, then it switches to the absolute time

• the branches list shows the time of the last commit to each branch

• the fetch button shows the relative time