Clear clipboard after login screen

[eloquence]

Description

Fixes #1051 by clearing the system clipboard contents after logging into online or offline mode.

Status

Ready for review

Test plan (online mode)

• Run the client from this branch, preferably in Qubes
• While the login screen is displayed, copy some string into the clipboard, e.g., "asdf" or your password
• Observe that the string you copied can be successfully pasted multiple times
• Log into the client
• Observe that the string you copied can no longer be pasted into the reply box

Test plan (online mode)

• Run the client from this branch, preferably in Qubes
• Copy some string into the clipboard, e.g., "asdf"
• Observe that the string you copied can be successfully pasted multiple times
• Click the offline mode option
• Observe that the string you copied can no longer be pasted (you may have to pick another window to paste into, e.g., a terminal)

Checklist

• Tested in Qubes. Qubes testing required.
• No AppArmor implications
• No migrations

[eloquence]

Not on the sprint, but I feel we may want to land a version of this alongside freedomofpress/securedrop-workstation#533, to put some guardrails around the vault password copying story.

[eloquence]

The duplication here is intentional; I felt that this behavior should not be a side effect of another function, but explicitly invoked so the intended business logic is clear.

[eloquence]

Added test plan & tests; will take a quick spin in Qubes now, as well.

[eloquence]

Tested both scenarios successfully in Qubes, including w/ cross-VM copying.

[redshiftzero]

diff LGTM, thanks for preparing it. Ran through the testing in Qubes and all works as advertised.