Rename KeePassX template filename to match default KeePassX filename and adjust docs

[huertanix]

Description

Currently, the included template KeePassX DB file in /tails_files is named securedrop-keepassx.kdbx and the documentation asks admins to copy and save this file to whatever they want to name it to in the documentation and save it wherevs inside ~/Persistent. The file should be renamed in the git repository to keepassx.kdbx and the docs should be updated to specifically ask that it be copied to the root of ~/Persistent so that the file is automatically loaded when KeePassX is started rather than having to ask users to manually hunt for a database file in a desktop environment they're unfamiliar with. Tails went through an effort to make this work so that users who are already new to password managers could have one less thing they have to manually deal with (See: https://redmine.tails.boum.org/code/issues/12375#note-18).

User Research Evidence

In observation of SecureDrop admins and journalists at SecureDrop trainings, many get frustrated with the amount of steps it takes to get to log in to the journalist interface. To remove this step and reduce the the confusion/surprise/"What do I do here", it should already be named the default/preferred KeePassX filename and located where Tails can load it automatically. Hunting for a DB file as a prerequisite to seeing your passwords is an alien process even for people using modern password managers.

User Stories

As an admin or journalist, I want to be able to log in to the SecureDrop journalist interface without having to try to remember what a KeePassX database file is or where I put it/what it was named.