Implement network activity indicator

[eloquence]

We've agreed that we want a simple, general network activity indicator in the client to represent the following three states:

• idle: the client is not actively communicating with the server (may be represented through the absence of an indicator)
• busy: the client is actively communicating with the server (queue as described in Add queue for server actions #224 is being consumed and/or automatic refresh is happening)
• error: an error occurred communicating with the server, user action required (queue is stalled). A separate error message will also be shown, scoped in Implement network connectivity error handling #391.

While we will aim to provide contextual progress indicators, we will likely not be able to do so for all user actions (at least at first); in addition, network activity may be related to automated actions (refresh) or background actions (download while looking at another source).

A single, global network activity indicator lets the user know at a glance whether there is activity (which tells them that they may still have to wait for a certain action to complete), or whether user action is required.

This issue is blocked on final UX work (see freedomofpress/securedrop-ux#32).

User Stories

As a journalist, I want to know that the client is busy, so that I know whether operations I have initiated are in fact underway.

As a journalist, I want to know if there is a network problem, so that I can resolve it.

[eloquence]

NB: This is currently deprioritized in favor of addressing just the error case (#391) and conducting field observations to make an empirical determination about the appropriate level of additional user feedback about network activity and network conditions.

[eloquence]

We may revisit in future, but closing for now in favor of a more limited sync activity indicator (#687).