Visual feedback and throttling of instant's saves

[jakub-g]

There are several problems with the current way we save instants.

1. When the save is in progress, we do not give any visual feedback (like processing indicator), so the user might click multiple times.
2. When the save is done, we change the icon from "disk" to "checkmark" but I've noticed this is not always the case -- perhaps it's the issue with GH saving stuff but didn't calling us back or whatever.
3. GH limits writes per user, and saving aggressively every couple of seconds may result in bot detection system entirely suspending (temporarily, probably until someone from support lifts the ban) the GH user account and logging him out of the app.

Marking someone as a bot means also that his account and all pull requests/issues ever opened by him will return 404 until he's unblocked.

Hence the TODO here is:

• add processing indicator while save is in progress
• deactivate and dim the "save" button for a certain amount of time after each save (say 20 seconds), and display a popup when hovered telling how much time one needs to wait

Not sure what's the underlying issue with 2) though.