Add more widget sanity checking
This is for https://github.com/vector-im/element-web/issues/15705
https://github.com/matrix-org/matrix-react-sdk/pull/5459 was unable to track down all the instances of where the issue happens, so this commit tries to do a more complete job.
Specifically, this replaces the getRoomId() function given widgets cannot reliably be referenced by widget ID in this way, and the store has been updated to handle a more unique widget ID for the store (just in case).
Further sanity checking has also been added to ensure that we are at least returning a valid result.
