Support for kibana 7.6.X

Hello everyone :)

I make the migration of the plugin for kibana 7.6.X. Feel free to use it or not.
This is the zip containing the patch : [kbn_network_7.6.X.zip](https://github.com/dlumbrer/kbn_network/files/4376228/kbn_network_7.6.X.zip)

The patch include:
- The migration for kibana 7.6.X
- Remove the background option to have the dashboard background (usefull to switch between light and dark mode)
- Fix some Eslint issues
- The update of the vis-network library

To apply it, you have to :
- clone the plugin
- checkout the commit a4a1cdbbae0b43886ca9966d4aca2268f059beac of the branch `7-dev`
- unzip the patch
- use this command to apply the patch : `git am --signoff -k < PATCH_PATH `
> _Replace **PATCH_PATH** with the path of the patch_
- execute the `yarn` command
- start kibana (remove the content of the optimize directory before if you are not in dev mode)

Start from the 7-dev commit a4a1cdbbae0b43886ca9966d4aca2268f059beac and inspect the attached kbn_network_7.6.X.zip patch. Apply it with git am, run yarn, and start Kibana 7.6.X after clearing optimize when needed; done means the plugin loads with the listed background, ESLint, and vis-network changes.

Written by the indexing model from the issue text.