Add queued action feature

[lorumic]

I propose this general-purpose action queuing feature. The idea originates from the "battle -> choose name -> battle" flow.

When a non-named user clicks on the "Battle" button in the home, they are asked to choose a name (and log in, if the chosen name is registered). After choosing a name/logging in, they need to click the "Battle" button again to do what they originally wanted to do. Ideally, the action that generated the name choice request should be executed automatically once the user has chosen their name.

So, with this change, a general mechanism for action queuing is added - and currently applied to the "Battle" button in the home. The new behaviour is: after finishing the name choice/login flow, the pending action is executed automatically, with no further clicks needed.