Add a bounded fallback on failed elections (#10988)

* Allow `pallet-election-provider` to accept smaller
solutions, issue #9478

* Fixing a typo

* Adding some more tests
Removing a seemingly outdated comment

* making it a URL

* Updating test name as per suggestion

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Updating documentation to be more explicit

And to follow the general guidelines

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Fixing formatting

* `Fallback` now of type `InstantElectionProvider`
Some cleanups

* Allow `pallet-election-provider` to accept smaller
solutions, issue #9478

* Fixing a typo

* Adding some more tests
Removing a seemingly outdated comment

* making it a URL

* Updating test name as per suggestion

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Updating documentation to be more explicit

And to follow the general guidelines

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Fixing formatting

* `Fallback` now of type `InstantElectionProvider`
Some cleanups

* Merging types into one type with generics

* Removing `ConstUSize` and use `ConstU32`

* cleaning up the code

* deprecating `OnChainSequentialPhragmen`
Renaming it to `UnboundedSequentialPhragmen` which should only be used
at genesis and for testing.
Use preferrably `BoundedOnChainSequentialPhragmen`

* Amending docs

* Adding some explicit imports

* Implementing generic `BoundedOnchainExecution`
Removing the deprecated `OnChainSequentialPhragmen`

* Use the right Balancing strategy

* Refactoring `onchain::Config`
Creating `onchain::ExecutionConfig`

* Merge master

* fmt

* Name cleanups after review suggestions

* cosmetics

* renaming `instant_elect` to `elect_with_bounds`
Other corresponding changes as per @Kianenigma

 feedback

* `BoundedOnchainExecution` -> `BoundedExecution`
And `UnboundedOnchainExecution` -> `UnboundedExecution`

* feedback from kian

* fmt + unneeded import

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: kianenigma <kian@parity.io>