Increase workspace size to 8000x8000

[knolleary]

We occasionally get asked to increase the size of a flow canvas.

It has been 5000x5000 since the start of the project and for some users, it isn't enough.

Generally I've felt that once you're using that much space in a single flow, you have other organisational options available to better structure your flows.

One proposal was to make the workspace an infinite canvas. That is still an option, but is actually quite hard to get right.

So far the sake of a quick fix, this increases the workspace to 8000x8000 - giving 2.5x more space.

[coveralls]

Coverage: 68.627%. Remained the same when pulling 0fdf426 on increase-workspace into 683ae4f on dev.