Don't allow installation of app to repos with unsupported languages

[ericwb]

Describe the bug
Precaution currently only supports scanning Python and Go, with JavaScript coming soon. However, Precaution has no reason to be installed to repos that use only languages Precaution doesn't support. For example, if the repo uses only Java, there currently is no point to installing Precaution.

We need some kind of compatibility check prior to adding our App to a repo. There is a way to determine the languages for a repo using this API.

https://developer.github.com/v3/repos/#list-languages

But I don't know whether we can tell GitHub to abort installation if the repo doesn't meet our requirements.

To Reproduce
Steps to reproduce the behavior:

1. Choose a repo that is all Java based
2. Install Precaution
3. Notice nothing prevents this and now Precaution does nothing but monitor PRs.

Expected behavior
Shouldn't install Precaution on repos unsupported by it.

Screenshots
n/a

Additional context
n/a