Update to golangci-lint that can apply fixes for some custom linter diagnostics
Update to golangci-lint that can apply fixes for some custom linter diagnostics

Replaces golangci-lint with https://github.com/dbraley/golangci-lint/tree/khan-fork hash 8a72db8a2a50,
which improves issues coming out of analysis based linters (such as custom linters) to offer
potential fixes for diagnostics given the fix replaces 1 or more whole lines with 0 or more, and only
a single block of text per diagnostic.
Also updates the vendor folder, and adds .idea subdirectory to .gitignore

Issue: https://khanacademy.atlassian.net/browse/INFRA-4051

Test plan:
run make deps in the webapp directory to recompile khan-linter
run ka-lint to assert still works correctly
make a fixable problem in a go file (such as a comment that is too long)
run golangci-lint run --fix {filename}
verify file has been fixed

Khan/khan-linter,Khan/khan-linter,Khan/khan-linter,Khan/khan-linter