refactor: create dedicated composable for proposing power

[wa0x6e]

Summary

Toward #939

This PR extracts the proposition power from the useVotingPower into its own composable, and avoid mixing voting and proposing power.

Merging this one first will allow #938 to use the final correct name, without filename collision

How to test

1. Everything should work as before
2. Create a proposal on a space you have proposition power, you should be able to create a proposal
3. Create a proposal on a space you do not have proposition power, it should show a message about insufficient proposition power.

[Sekhmet]

tACK