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The way BLOBs are written to the database has changed in python3
So we read all BLOB values cast them to BLOB then write them back to
the table

Fixes #402, #278

Fixes #399

This commit contains all changes starting with the first integration of
the plugin into the gajim_1.1 branch (76dabe2e) until the current plugin
version V3 (42b9aeb8).
The manifest.ini is updated for compatibility with upcoming Gajim
versions.