[gatsby-source-contentful] Flight mode

[uttrasey]

Description

Add a "flight mode" option so it would be possible* to develop locally using Contentful CMS even if you have no network connectivity.

* Dependant on existing prior connection/cache

Actual result

Currently develop falls over

source and transform nodesAccessing your Contentful space failed. Perhaps you're offline or the spaceId/accessToken is incorrect.

Expected behavior

The app runs using nodes from last sync

[uttrasey]

If this is sensible I'll knock a PR together.... about to spend a lot of time on ✈️'s

[calcsam]

This would be great! Let's make the gatsby development experience on bad / spotty connections as good as the gatsby site viewing experience :)

[uttrasey]

Got around to doing this #3977

[KyleAMathews]

Due to the high volume of issues, we're closing out older ones without recent activity. Please open a new issue if you need help!