Add the new conditions and phrases for usage instruction

[marla-singer]

Closes #1759
Closes #1762

Implemented the scenario:

1. If Proxy backend configuration is completed, "Disable API key" is false and user haven't got api key yet, usage instruction has message as "use your API key. Click the "Get API key" button to request the key"
2. If Proxy backend configuration is completed, "Disable API key" is false and user has got api key, usage instruction has message as "use your API key in the URL parameters or in the request headers"
3. If Proxy backend configuration is completed, "Disable API key" is true, usage instruction has message as "use the URL"

[marla-singer]

@brylie Could you please review it?

[jykae]

Reviewing

[jykae]

@marla-singer works as described, what you think, could API key itself be also hidden from API view if "Disable API key requirement" is true for API? It could probably done easily with your new helpers. We can also put it as enhancement, as it is not described issue/task.

[jykae]

@marla-singer We discussed this in daily, I merge this one and we'll do above thing if we get user feedback about it.