Updates Source Interface docs language
Updates Source Interface docs language

Makes two updates to the text of the Source Interface in order to match
the new screenshots:

Renames Source Interface button "Logout" -> "Exit" in docs

The name of the "logout" button as presented in the Source Interface
changed back in #1534, but the corresponding documentation was never
updated. Now that the screenshots show the new "Exit" button, we must
also update the guiding language to refer explicitly to an "Exit"
button, not a "Logout" button.

Updates Source codename displayed in-line to match the screenshots.
Since Source codenames are randomly generated, updating the screenshots
means we'll definitely get a new codename, so let's update the in-line
text to match what's displayed in the visual aids, to avoid confusion.

conorsch/securedrop,heartsucker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,garrettr/securedrop,conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop,micahflee/securedrop,garrettr/securedrop,micahflee/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,garrettr/securedrop,micahflee/securedrop,conorsch/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,micahflee/securedrop,conorsch/securedrop,heartsucker/securedrop