Crowdloan contribution memos can be overwritten

[JoshOrndorff]

The crowdloan pallet provides the opportunity for contributors to add "memo" to their contribution. This feature was added in #2728. The memo is never read or used by the relay chain itself, but it is stored in relay chain storage. This field provides a way for contributors to trustlessly associate a parachain identity with a contribution in order to reap rewards on the parachain. (The reward scheme itself is left entirely up to the parachain).

The existing add_memo extrinsic can be called multiple times for the same contribution with each call overwriting the memo from the previous call. This makes it challenging for parachains to know which memo to use, especially in a trustless way.

consider a scenario where a user adds a memo, then the parachain issues one-time rewards, then the user changes the memo on-relay-chain. Of course there are any number of concrete ways to address this (take the first memo, take the most recent memo at the time of reward initialization). But it adds complication, and opens the possibility for different parachains to handle it differently from one another, leading to further confusion in the ecosystem.

I recommend that this extrinsic be changed to check whether a memo has already been supplied, and, if it has, return an error, not overwriting the old value.