LSIF: add a config setting to control checking of LSIF upload tokens
Created by: chrismwendt
Prior to this PR, the LSIF upload token validation could not be bypassed, even through site config. This made it impossible to upload LSIF for non-GitHub repositories (there's no supported way to prove repository ownership).
After this PR, LSIF upload token validation can be toggled by the boolean site config lsifEnforceAuth
. By default, all LSIF uploads will be allowed. Sourcegraph.com will have this set to true, meaning LSIF upload tokens are required. This makes it much easier for customers to try out LSIF on their own instances.
Resolves https://github.com/sourcegraph/sourcegraph/issues/5516
Test plan: manual while LSIF support is in alpha