Remove redundant pip install and add install for sapp

[gbleaney]

Issue #444 pointed out that SAPP is missing from the venv when doing the tutorial. This is because we removed pyre-checks dependency on fb-sapp, so that users of Pyre wouldn't have to bring in a whole web server when all they wanted was a type checker. This change updates our Dockerfile and README.md to now explicitly install fb-sapp.

Additionally, we've fixed the dependencies on the fb-sapp package, and we no longer need to explicitly install dependencies like flask or click. For that reason, I've also removed the now redundant extra pip install lines.

Test Plan:
Removed my old venv:

rm -rf tutorial/

Set up a new one with just pyre-check and fb-sapp:

pip3 install pyre-check fb-sapp

Ran exercise 4, which requires both:

pyre analyze --no-verify --save-results-to .
sapp analyze taint-output.json
sapp explore

Was able to interactively explore models in SAPP

[facebook-github-bot]

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[abishekvashok]

LGTM

[facebook-github-bot]

@gbleaney merged this pull request in 40cf5b0.