`nomination-pools`: add permissionless condition to `chill` (#3453)

Currently, pool member funds cannot be unbonded if the depositor's stake
is less than `MinNominatorBond`. This usually happens after
`T::MinNominatorBond` is increased.

To fix this, the above mentioned condition is added as a case for
permissionless dispatch of `chill`. After pool is chilled, pool members
can unbond their funds since pool won't be nominating anymore.

Consequently, same check is added to `nominate` call, i.e pool can not
start nominating if it's depositor does not have `MinNominatorBond`

cc @Ank4n @Kianenigma

 

closes #2350

Polkadot address: 16FqwPZ8GRC5U5D4Fu7W33nA55ZXzXGWHwmbnE1eT6pxuqcT

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Co-authored-by: Ankan <[email protected]>
Co-authored-by: Gonçalo Pestana <[email protected]>
Co-authored-by: command-bot <>