Generic-fy Phragmen into Election

[Swader]

Looks like we're dropping Phragmen in some big ways due to the difference between the custom implementation and the original, so we'll need to update the phragmen page to reflect this some time soon: paritytech/substrate#6886

[salmad3]

Let's evaluate this, will also address: #2149

[salmad3]

https://wiki.polkadot.network/docs/learn-phragmen

needs some investigation - we can reach out to the right folks to clarify @a-jwc