Include KeePassXC template database in Debian

[ChumOfChance]

Description

On setting up Admin and Journalist workstations, currently users are advised to copy a template KeePassXC database that is include in the cloned repository over to the Tails persistent storage: https://docs.securedrop.org/en/stable/admin/installation/set_up_keepassxc.html

This is awkward, because we now only use the repository to bootstrap the installation of the Debian package. It is preferable to not clone the repository to Tails persistent storage to minimize user confusion: the repo is only needed for initial installation, so it should be deleted (nuked by Tails reboot) afterwards.

This PR attempts to address this and fix the documentation for the current situation, but if the KeePassXC database was included in the Debian package and installed directly to ~/Persistent/ then it would be readily available for users. This would apply to both Admin and Journalist Workstations, and would allow simplification of the documentation.

Making this change will also be applicable to a future without Tails, where SecureDrop admin tooling is installed on Qubes only.

Also worth noting that Tails intends to migrate from KeePassXC to GNOME Secrets, likely in the next version (7.5) so very soon. See #7756

[zenmonkeykstop]

Probably worth rolling in two changes here:

• add installation of keepassxc (likely just as a dep of securedrop-admin package)
• add keepassxc template to securedrop-admin package, and maybe install it somewhere accessible as part of the package install (otherwise users would have to copy it from /usr/share/securedrop-admin/templates or whatever, which is not the end of the world.)

[ChumOfChance]

I'm in the middle of creating another issue to track the installation of KeePassXC if GNOME Secrets arrives. Should I still do that? Two changes, two issues? Or you wanna keep it all together here.

[ChumOfChance]

Two changes two issues #7756 2️⃣