Don't re-download required plugins, update community plugin list.

[miceg]

Some more prep work as part of #642:

• Create required_plugins.txt which lists all mandatory plugins.

  A future PR will use this file as a download list.

• Remove all mandatory plugins from stable_plugins.txt, which are already downloaded elsewhere.

• Add community plugin wfs-freemarker-plugin.

• Remove community plugin rat-plugin (doesn't exist anymore?).

[aaime]

The rat community module graduated to extension in 2.25.1