Implement proxy functionality

[traumschule]

This may become an epic on how important the teams takes the topic.

Related: Joystream/joystream#5029

Problem

1. I added an 'any' proxy to my membership account. (block)
2. I hide the main account in my wallet.
3. Load Pioneer only with the proxy active.
4. Expect the original membership to be found and loaded <- not the case
5. Add functionality to (un)set proxy via Pioneer's Edit membership modal.

Test case: Which type applies for permission to post on the forum?
Sidenote: the handbook references this wiki page explaining proxy types but it doesn't contain all types available on our chain.

Step 4 likely requires a reverse lookup or QN update.

[mochet]

@traumschule you might be able to add the account as a "watch only" account within your wallet (some of them support this). That might help.