Use an existing account for the me-Screen (IOS-255) (#1310)

Technically this is a workaround: Replacing the `ProfileViewModel!` with
a real optional (or no optional at all) most probably requires a rewrite
of the entire Profile-screen, including header etc. as the
`ProfileViewModel!` is used basically everywhere.

So the workaround is to use the existing account (at least until it is
updated and replaced) to not have the app crash when the user taps on
their profile too quickly (without an account, which this PR fixes). For
some reason, we persist the users very own profile, but never used it.

Mastodon/Scene/Root/MainTab/MainTabBarController.swift

@@ -85,6 +85,10 @@ class MainTabBarController: UITabBarController {
             notificationViewController.viewModel = NotificationViewModel(context: context, authContext: authContext)
             homeTimelineViewController.viewModel = HomeTimelineViewModel(context: context, authContext: authContext)
             searchViewController.viewModel = SearchViewModel(context: context, authContext: authContext)
+
+            if let account = authContext.mastodonAuthenticationBox.authentication.account() {
+                meProfileViewController.viewModel = ProfileViewModel(context: context, authContext: authContext, account: account, relationship: nil, me: account)
+            }
         }
 
         super.init(nibName: nil, bundle: nil)