FES integration to retrieve Client Configuration (OrgRules)

[tomholub]

part of #275

1. steps 1-5 must be done first from issue prepwork for retrieving client configuration (OrgRules) from FES #339
2. remove toast message from step 5 from issue prepwork for retrieving client configuration (OrgRules) from FES #339
3. instead of showing toast, call GET <fes>/api/v1/client-configuration?domain=example.com (no authentication). The response is { clientConfiguration: DomainRulesJson }. If this step was not skipped (because FES was not running at all earlier when testing /api/ and this call fails, show error and don't let user proceed unless they can successfully retry this call.
4. Save this OrgRules json into storage, separately for each account. If it was in storage for this account before, blindly overwrite the previous OrgRules.

Here's an example: https://fes.flowcrypt.com/api/v1/client-configuration?domain=flowcrypt.com

[tomholub]

Also add the following class ported to Swift. It's used for parsing the OrgRules JSON and offering methods that make the purpose and status of each rule more obvious for later use. The class won't actually be used anywhere yet (use of this class in the rest of the app will be done in a followup issue). https://github.com/FlowCrypt/flowcrypt-browser/blob/master/extension/js/common/org-rules.ts

[tomholub]

You can use your @ flowcrypt.com email address to test this. This will call fes.flowcrypt.com where there's an instance running.

[tomholub]

Let's do this after #310

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