refactor: revamp settings and migrate to overmind

[Spring3]

Re-shaped the structure of settings that the application stores locally.

Migrated settings to overmind, as well as to the main thread only. So, with this PR render thread won't try to access settings directly, but instead will request them from the main thread, following the same pattern as http requests.

The data structure of the settings (at the time of writing, will look like the following):

{
  activeSession: {
    user: {
      id: '',
      firstName: '',
      lastName: '',
      createdOn: '',
    },
    endpoint: '',
    hash: '',
    settings: {
      ...
    }
  },
  persistedSessions: [{
    user: {
      id: '',
      firstName: '',
      lastName: '',
      createdOn: '',
    },
    endpoint: '',
    hash: '',
    settings: {
      ...
    }
  }, {
    user: {
      id: '',
      firstName: '',
      lastName: '',
      createdOn: '',
    },
    endpoint: '',
    hash: '',
    settings: {
      ...
    }
  }]
}

Things to do:

• verify that the sessions are saved when logged in / out
• verify that the last active session gets restored and the user doesn't have to log in repeatedly