Error loading settings

Closes #542 (closed)

Commit "Display error when settings can't load": Before, when an error occurred trying to load account settings, the user would see a spinner until pressing the "go back" button manually.

With this commit, when an error occurs trying to load account settings, an error dialog opens. Furthermore the "go back" button is activated automatically. That is done via the new function activate_action in fractal-gtk/src/actions/global.rs. The reason for having that function globally and not implementing it for AppOp is to avoid trying to lock the same mutex twice in the same block: once with the macro APPOP!(see line 34 of fractal-gtk/src/app/mod.rs) and once in the activation of the simple option 'back' (see line 266 of fractal-gtk/src/actions/global.rs).

In particular, with this commit, when the user closes the error dialog, they find themselves back in the state they came from.

Furthermore, the functions that used to be called show_three_pid_error_dialog and show_error_dialog are refactored a little bit in this commit.

Commit "Fix deserialization of enum Medium": The serde attribute we want is "rename_all" rather than "rename".