Add tooltips for subtasks and description.
Implement LDAP user lookup.

This is required to improve compatibility when the DN cannot be easily
computed from the user name. Additionally, this allows automatically
getting the full name and email address from LDAP.
Allow 'no category' and 'unassigned' in actions.

This can be used to assign a color to task with no category/assignee, or to reset
assignee.
Use popover to edit task and description directly from board.

- Click on 'description' icon opens the 'edit-description' page.
- Edit task and edit description now open in popover, like the assign
dialog.
Use shorter date display, and add icon.
Display icons on the left of counters.
More compact task board.

- Move category indication to top-right corner.
- Rounded corners.
- Slightly increase icons size.
- Slightly reduce font size.
- Reduce title margin.
Add option to disable SSL certificate verification for LDAP.