* set contact to inactive if it becomes offline

* send idle also in case app started in background

closes #2937

Thanks to @tmolitor-stud-tu for explaining the situation in
https://github.com/guardianproject/NetCipher/pull/74

https://firebase.google.com/support/guides/disable-analytics

Conversations stores sensitive data, such as SQLiteAxolotlStore tables, in its database.

secure_delete=ON is the default for newer Android devices:
https://android-review.googlesource.com/c/platform/external/sqlite/+/209123

However, older devices had it disabled by default, so any database created than has this setting disabled and it will not be updated automatically.

Also, the default might be changed in the future (for example, to "FAST", added in 2017), so it is safer to set it explicitly.

This way we avoid repeating the code. Unlike getWritableDatabase, onConfigure is intended to be overridden.

In fact, onConfigure documentation specifically says: "Called when the database connection is being configured, to enable features such as write-ahead logging or *foreign key support*."

Perform case-sensitive matching for highlighting the own nickname in MUC
messages and while checking whether to notify.

* fix a missing word in README

* fix repeating word in README

Currently QR scanner supports only URIs. VCard specification allows
embedding instant messaging protocols in the `IMPP` field [0].

This change will extract the first XMPP URI from `IMPP` field if a VCard
has been scanned and process it just like if the XMPP URI was scanned
directly. In case the contact is not already present in the roster this
will pop up "Add contact" window.

Example VCard with this URI:

    BEGIN:VCARD
    FN:Test Contact
    EMAIL:test@example.com
    IMPP:xmpp:test@example.com
    END:VCARD

[0]: https://tools.ietf.org/html/rfc6350#section-6.4.3