nomination-pools: prevent bonded_balance to go below MinJoinBond

[kianenigma]

Currently, once a member wants to join a pool, we enforce them to put at least MinJoinBond into the pool.

substrate/frame/nomination-pools/src/lib.rs

Line 1422 in abb80e5

ensure!(amount >= MinJoinBond::<T>::get(), Error::<T>::MinimumBondNotMet);

But we allow them to immediately unbond and go below this value, without any checks. To fix this, the unbond transaction should ensure that the bonded balance is still more than MinJoinBond, or it is 0.

Note that bonded balance of the member needs to be computed and is not currently know at the scope of fn unbond. Nonetheless, the conversion is possible. All points are always convertible to balance via the points_to_balance function.

Lastly, since all transactions are now transactional by default, it is easiest to check this condition at the end of the function, after member has been updated, potentially using member.active_balance (which is correct but slightly inefficient).

Once done, the documentation of MinJoinBond should be updated, and proper tests should be added.