Limit number of active proposals by a member

[mattsse]

Issue summary

There are no limits on how many proposals can be submitted.
To mitigate spam a threshold is required that caps the number of active proposals by a single Account

Possible solution:

• extend the Proposal type with the submitter.
• refactor the Members map to also include a counter of submitted and active proposals
• increase proposal counter if a new proposal was submitted
• decrease proposal counter if the proposal was closed

Other information and links

• #372

[mattsse]

this feature is currently not required as it goes against the expected usage of proposal submissions