[Lens] Remember last used index pattern on startup

[timroes]

Lens should remember the last used index pattern (at least within the browser) when starting the app.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

We have talked about this a bit, and it seems like this is more like an editor preference that should be stored in localstorage, so that each user can resume from their previous state.

It does not belong in the URL because index patterns can change when going from Lens -> Dashboard and back.

[wylieconlon]

When opening a previously-saved Lens object in the editor, we already open the index patterns that were being used. As long as this behavior doesn't overlap with this case, I think the fallbacks are:

1. Use the saved doc if available
2. Use the localstorage saved index pattern
3. Use the default index pattern