Add expireable proxy, that spends from the main account.

[siman]

• Specify the max tokens that can be spent by the proxy
• Set a time to live for the proxy, beyond this time, the proxy will lose access.

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Alex:
We should allow a proxy to spend tokens of the main account to pay txs fees.
We can implement a transaction extension that will handle this.
An important thing here is to specify the maximum amount of tokens per a certain period (a day?) that can be spent by your proxy accounts.

Bryan:
a quota could be enough. the fee is usually relatively cheap, and the user should rotate this proxy every few days, so I don't think a cap per day is needed

Alex:
Ok, then the dapp should ask a user to create a new proxy for example every week.

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Related issue: https://github.com/dappforce/private-subsocial-web-app/issues/245