Make codeIntel.lsif true by default

[sqs]

After you upload LSIF data, you need to remember to set codeIntel.lsif to true in settings. This step seems unnecessary from the user's POV. Why would a user want to default to not using LSIF if it exists? I can see edge cases where they would want to disable LSIF (perhaps they're testing something, or someone uploaded bad LSIF data), but those seem like just that: (rare) edge cases.

If there's concern about perf impact, then at the very least we should add a notice to the repo settings LSIF pages telling the user to set codeIntel.lsif to true if it isn't already.

Related: Slack discussion from 2020-09-03