Code nav: single-click jump-to-definition

Created by: chrismwendt

This lets you jump to definition by clicking on the token:

You can still select text even though the cursor is the hand icon 👆

This is a prototype and the plan is to merge it behind a feature flag to get a feel for the UX.

Motivation

This is similar to how Bazel and Haskell Code Explorer work and was requested by a user.

Feature flag

Global/org/user settings:

  "singleClickGoToDefinition": true,

TODO

• Merge https://github.com/sourcegraph/codeintellify/pull/106 (depends on a new API)
• Figure out how to avoid needing to assign to window.location (currently causes a full page load when jumping to a different file)
• Put this behind a feature flag

See proposal doc https://sourcegraph.slack.com/archives/CHXHX7XAS/p1555110738009200