Add table panel for grafana version 7

[BastienF]

Reopen of #305 because of branch cleaning on source repo.

Fixes #236
If you see a better solution than creating a specific libsonnet file, let me know I can probably code it.

Because of the complete rewriting of table panel in grafana 7 the solution used in other libsonnet file (to have a pluginVersion attribute to switch between grafana versions) is not viable for this panel.
So I've added a brand new libsonnet file that should live along with the old one (table_panel.libsonnet) to allow retro compatibility

This file is a copy of https://github.com/BastienF/grafonnet-lib/blob/master/grafonnet-7.0/panel/table.libsonnet

[roidelapluie]

Why aren't you using grafonnet-7.0 directly?

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❌ Bastien Fiorentino
❌ BastienF

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