content.json: File permissions / Rate limiting

### Suppose:
1. User can modify their own `data.json`
2. `data.json` contains list of messages (e.g. forum posts)
3. User can add as many posts as they want since they don't have any restrictions
4. ???
5. FLOOD or MODIFYING POSTS AFTERWARDS!11

## Possible solutions:
### Storing history of file modifications with timestamp
#### Problems:
1. Need to verify timestamp somehow
2. Need to implement history/blockchain
3. 51% attack

### ~File per post + Rate limiting + File permissions:~

1. Allow to add one file per `content.json` update
2. Rate limit modification of `content.json` (check via `modified` key)
3. One file contains one post
4. User can only create files, not modify

#### Problems:
~1. Many files~
1. We can't rely on `modified` timestamp

### ~Smarter ways to check modifications of JSON data:~

1. Ability to restrict addition of one item to `data.json` list per `data.json` update via `content.json` rules
2. Ability to restrict items modification
3. Rate limit modification of `data.json` or `content.json`

#### Problems:
1. (Possible) DoS attack via big JSON files (each node should check modification of files)
2. If all nodes is offline, you can create as many posts as you want

### ~Maybe even ~JS~ CoffeeScript/Python scripts to check files modification~
#### Problems:
1. Kinda dangerous (sandbox's required)

Start by reading the issue's proposed content.json and data.json update rules, including the file-per-post and rate-limiting alternatives. Determine which abuse cases and trust assumptions can be specified, then document a decided approach and its verification criteria; the issue currently does not identify implementation files or tests.

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