Update SecureDrop screenshots for source, journalist and admin guides

[eloquence]

This also includes a correction in the docs that describe a screenshot, which refers to the "Exit" button (it was once briefly called that, but is now back to "Log out").

Resolves #4529

Status

Ready for review

How to review

I recommend using GitHub's rich diff feature to ensure that the content of each screenshot matches the one it replaces. Click the "rich diff" icon for each file on the "Files changed" tab:

Checklint

• make docs-lint passed locally

[eloquence]

I used @rmol's sceenshots fixes to work around a bug in develop (#4558) where all screenshots by the functional tests produce tofu instead of legible characters. It saved a fair bit of time, but I still had to manually crop several screenshots, and retook some to capture the right target area.

Note I forced the version string to 0.14.0 for a cleaner reading experience.

Even with John's fixes, my first run for the admin UI screenshots produced a lot of screens without the logo so I'm not including those in this PR; will do another run to see if that's flakiness or consistent.

[eloquence]

With the changes in #4559 and more manual editing/re-taking of some screenshots, I was able to update the admin guide screenshots as well.

[redshiftzero]

LGTM, thank you!