move all ios-app related folders/file into a subfolder

[tomholub]

our project structure is becoming more varied, and it's becoming a bit of a mess on the root folder:

• there's the Core folder (TypeScript)
• appium folder (TypeScript / for running UI tests)
• fastlane folder
• docker-mailserver (currently unused)
• Scripts
• many folders for the app itself
• now I'm just about to add a mocks server for Create a FES mock for UI tests, make the tested app use it #1006

I think we should group things together more, and the one thing that needs grouping the most seems to be the app source files themselves. I recommend to create a folder app where FlowCrypt, FlowCryptUI, FlowCryptCommon .. etc would go. Maybe even the project files, a Podfile, Makefile, Gemfile, swiftlint.