Flow pallet major changes

[vovacha]

Refactoring reasons: SBP-authored issues (storage, error handling), common naming convention (state names, errors, events, etc.)

Flow new features:

• Added optional start parameter for create_campaign and scheduled campaign activation flow: Add start block to campaign creation  #71
• Added MinCampaignDeposit (first iteration: 10% of target and 1:1 GAME-to-PLAY conversion). Next iteration with oracles.
• Added CampaignDurationLimits. Runtime configured using 24 hours - 60 days interval.

Flow refactoring:

• Campaign settlement logic was rewritten (different approach, no more Finalizing and Reverting states).
• Removed update_state extrinsic, discussion -> Flow pallet #84 (reply in thread).
• Complexity for create_campaign reduced: O(log N) -> O(1).
• Campaign id generation: hash of campaign object instead of hash of nonce (same approach for proposals).
• Changed type Campaign.created Timestamp -> BlockNumber, discussion -> Unixtime #82 (reply in thread)
• Removed redundant storage, discussion -> SBP: Storage usage #53 (comment) and SBP: Flow #56
• Removed redundant errors.
• Error handling changes due to SBP: Error Handling #54
• Naming changes (campaign states look similar to proposal states now, event names, storage names, etc.)
• Benchmarks, tests, mocks.

Signal minor changes:

• Mostly validation changes and fix for Withdrawal proposal actions.

Discussion:

• How to handle the old migrations @vasylenko-yevhen ? I've commented it for now.
• There is no single approach dealing with on_finalize and on_initialize hooks. Currently, Signal uses mostly on_finalize for calculations and Flow, on the contrary on_initialize (probably I'll create a discussion).

TODO:

• Transactional macro is broken, some of the tests were deactivated. Already raised a question: https://substrate.stackexchange.com/questions/4267/transactional-macro-deprecated-but-still-required
• Flow's on_initialize benchmark validation has some weird issues.