IOCs watchers' sources initial management from backend

[securechicken]

Added: Web backend can now add/delete IOCs watchers sources in config.

In the process, tuned "write_config" in server/backend/app/classes/config.py so that return's doc description is fixed, return messages are more explicit (notably for IOCs watchers sources writing), and IOCs watchers sources URL is simplified. Few changes there, but shown as much because of whitespaces: diff must be viewed with whitespaces ignored.

[felixaime]

I'll test and push it. But I've no real internet link, maybe on Friday. Thanks, Félix.

[CLAassistant]

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