Fix forwarding immediate view, use `issued_inbox_tasks` tab.

Fixes #306.
Merge pull request #307 from 4teamwork/deif-fix-forwarding

Fix forwarding immediate view, use `issued_inbox_tasks` tab.
Merge pull request #309 from 4teamwork/deif-oguid-with-adminunit

Reworked oguid
Add `SchemaMigration` base class for sql-database migrations.
Rewrite existing DB-migrations to use the new `SchemaMigration` base class.

Closes #296.
Remove file from version control.