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Otherwise, these fields cannot be emptied by the user

fix #3281

fix #3285

fix #3224

fix #3274

fix #3277

Use a db roster to manage biboumi’s presence with the contacts

Closes #3217

See merge request !13

By using SFINAE to use that member only when it exists.

Everything was already cherry-picked, or something like that, so no changes
are involved, at all.

If we can’t open it at startup, we exit.
If we can’t open it on reload, we keep the previously-opened database.

This way, we’re assured to always have a valid and open database available.

fix #3269

ref #3269