Add mypy to the webapp

[DavidKorczynski]

We recently had a regression that caused the webapp to fail after a sequence of API calls were made: #1559

I think we could have caught this using strict type checking since

fuzz-introspector/tools/web-fuzzing-introspection/app/webapp/routes.py

Line 1027 in 886fdbb

all_functions.append(data_storage.get_constructors())

would have been caught by type analysis, as the line appends an element of type list, which is not the type of element the list holds.

[DavidKorczynski]

@arthurscchan

[arthurscchan]

The CI has discovered serveral existing problems and bugs in the web apps and they have been fixed before introducing this new CI. The fixes for these problems and bugs are merged in PRs #1564 #1565 #1566 #1567 #1568 #1569 #1570 #1577.