Runtime: Polkadot Fellowship promotion/demotion periods, members activity and salaries (#2607)

* core fellowship

* core fellowship weights

* salary

* weights

* fellowship pot

* registration period 15 days

* use treasury account for salary pay, promotion origin

* decision period for tracks 30 days

* docs

* comment

* Couple of fixes and some refactoring

* Alter curves to be a bit more conservative

* Use `PayOverXcm` for fellowship salary payments

* Docs and remove unneeded code

* Fixes

* Move Fellowship stuff in line with whitepaper

* fix: induction by a single Fellow (not proficient)

* doc fix

* renames, pallet index, allow unpaid for salary pallet

* Fix budget units

* Fixes

* Test sovereign account for Fellowship salaries

* Nice address test

* Fixes

* test for PayOverXcm setup

* Update parachains/runtimes/collectives/collectives-polkadot/src/fellowship/mod.rs

Co-authored-by: Bastian Köcher <[email protected]>

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Co-authored-by: Gav <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>