WebDAV support

[anon8675309]

As @c4f3a0ce mentioned in #17 (comment) there is a desire to have support for vendor-independent storage. Issue #17 is about offline functionality, so the discussion of additional online functionality should be its own ticket (this one). These are related, but distinct feature requests.

Trezor users are, generally speaking, strong supporters of self-reliance. This means having the ability to self-host one's own data. It means having access to the source of the software you are relying on, and not being locked in to just a handful of centralized providers. Most of the community will not object to having the option of using public file hosting, but I don't think any of us would object to supporting alternatives.

WebDAV is the de facto standard for syncing, especially on mobile devices. Desktops also widely support the standard, and server software such as Nextcloud can provide this service. Many crypto users already have a trusted WebDAV server to sync their contacts, calender entries, events and files, thus it makes sense to also store their password database there as well.