Fix Flow warnings

[jessebeach]

We had numerous Flow type warnings in the repo which were making PRs hard to review. This cleans up all the warnings.

[codecov]

Codecov Report

Merging #785 (80da3d0) into master (ecec8e4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 80da3d0 differs from pull request most recent head 426d4c2. Consider uploading reports for the commit 426d4c2 to get more accurate results

@@ Coverage Diff @@ ## master #785 +/- ## ======================================= Coverage 99.24% 99.24% ======================================= Files 98 98 Lines 1452 1453 +1 Branches 477 477 ======================================= + Hits 1441 1442 +1  Misses 11 11 

Impacted Files	Coverage Δ
__tests__/__util__/ruleOptionsMapperFactory.js	100.00% <100.00%> (ø)
src/rules/anchor-is-valid.js	100.00% <100.00%> (ø)
src/rules/control-has-associated-label.js	100.00% <100.00%> (ø)
src/rules/interactive-supports-focus.js	97.05% <100.00%> (ø)
src/rules/label-has-associated-control.js	93.18% <100.00%> (ø)
src/rules/media-has-caption.js	100.00% <100.00%> (ø)
...s/no-interactive-element-to-noninteractive-role.js	100.00% <100.00%> (ø)
...rc/rules/no-noninteractive-element-interactions.js	100.00% <100.00%> (ø)
...s/no-noninteractive-element-to-interactive-role.js	100.00% <100.00%> (ø)
src/rules/no-noninteractive-tabindex.js	96.77% <100.00%> (ø)
... and 3 more

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