Clarify error when user attempts to generate codename in tab2 after logging out in tab1

[eloquence]

During QA for SecureDrop 1.3.0, @emkll reported the following session handling behavior:

There's an edge case, though not sure it's worth addressing:

1. Click "Get Started" or hit /generate in both tabs A and B
2. Click "Submit Documents" in tab A and logout
3. Click "Submit Documents" in tab B, get immediately redirected to "You were logged out due to inactivity".

This behavior is new to 1.3.0, as prior to #5075, you would get an error on step 1.

For reference, the "Submit Documents" button is displayed on this screen:

Questions:

• Would it be preferable to let the user proceed to the submit screen? The downside is that it may result in the confusing generation of a second codename. On the other hand, that may be what the user wanted to do.
• If we do bounce the user back to the index page, should we clarify the error message and explain what happened (something to the effect: you logged out in another tab, try logging in again with your codename?)

[eloquence]

Tagged with security as this is a sensitive part of the source experience to make changes to. At the same time, as Mickael noted, this is an edge case, and IMO relatively low priority.