Cleanup duplicate logic and refactor installations.js into InstallationsRouter.

[nlutsenko]

Depends on #334.

• Reuse duplicate logic instead of writing it twice
• InstallationsRouter is a subclass of ClassesRouter and calls into super class methods

@drew-gross, what do you think about this approach over the custom function-based classes implementation that you have?
It's super clear what calls into what if we follow this approach.

Side-question - do we want/need to handle the same logic for body.keys/relations (redirectClassNameForKey) as for all other Classes?
Asking, 'for a friend', because we don't seem to do it right now.

[facebook-github-bot]

@nlutsenko updated the pull request.

[drew-gross]

This also works and is still pretty readable, although with no member variables the advantage having any type of classes, functional or not, seems pretty minimal. Calling super.handleFind doesn't seem any better than exporting a handleFind function from ClassRouter and calling 'ClassRouter.handleFind. I guess if you want to insert another subclass betweenClassRouterandInstallationRouter` it could help, but an equivalent can also be arranged using standalone functions. As always I don't want to impose my personal style so even if this isn't the exact way I would write it, feel free to merge regardless :)

[facebook-github-bot]

@nlutsenko updated the pull request.