Database upgrade, persistent connections and bulk metadata editor

Today's update (a8632bd) brings with it the following changes:

Performance improvements

The database system has been updated to PostgreSQL 10. This should give numerous performance improvements.
Persistent database connections feature has been implemented. This should lower load on the database system during high-traffic situations.
Certain features that would require database locking, like updating user's last activity timestamp and object last read timestamp is now done in a single atomic database call, getting rid potential race conditions and locking issues.
Improved performance when listing contents of inbox/outbox.

The CodeMirror and TinyMCE editors now have a word counter.
It's now possible to edit metadata for all containers and objects in a container in a simple user-interface. You can access this feature in the Edit menu when viewing a container. You can use Ctrl-Up/Down/Home/End to more quickly navigate the input fields in the table.

When adding users to institutions using the table or text user interface, our standard CSV parser is now used.
It is now possible to not inform the supervisor when an assignment is delivered. You can enable this feature by setting the custom_init variable dont_inform_supervisor=1 on the hand-in object.
Exception handling has been completely reimplemented, which should give more sane error handling.
Some database queries have been moved out into external files to make it easier to develop them further. Some of them now use newer PostgreSQL 10 features.