Implemented database migrations for mentat_events database.

The database migrations for IDEA event database *mentat_events* are now implemented. I have moved this issue to the next release, because there is code already merged in the current development branch that depended performing the migrations on target system. It was a choice of either removing that commit, or going forward with implementation. I have decided to implement it immediately, because this issue is blocking many more features.

The implementation is again based on Alembic tool, which is already being used for migrations on mentat_main metadata database. It is however separatelly configured instance, merging both instances into single one would require considerably more amount of research and work.

I have also updated accordingly manual pages regarding installation and upgrading. Very important bit of information was stamping the database with latest migration revision after clean installation.

(Redmine issue: #4230)