Make more of lab manager dependencies optional

[vidartf]

It would make the lab manager more reusable if it had fewer strict requirements. I kept the rendermime dependency strict, as I see no way to use it without this.

Other things that I added since they were related (see individual commits for split):

• Cleaned up use of types.
• Fixed an assign in if clause (should have been compare ===).
• Added strict null checks to lab manager package.

If any of these auxiliary points are contentious at all, just say the word and I will split up the PR.

[jasongrout]

At first glance, this looks great as an evolution of the codebase (we're slowly getting stricter typing)!

[vidartf]

Let's wait on a rebase after #1734 to make this diff smaller...

[vidartf]

I cleaned up this PR a lot, so it should now be a lot quicker to review!

[jasongrout]

Thanks!