Disable single joins for low liquidity pools

[groninge01]

The single asset join does not work when the single asset is > 25% of the pool liquidity.
Detect this case, block single join and show a warning with a clear explanation.

If single asset join AND input > 25% of pool's token balance

Block action
Show warning "You can only add up to 25% of a single asset to the pool in one transaction"