relationships in progress
property type 'association'

What is done: havind property of type 'association' and displaying
links to others issues (forward only)

Missing:
- compute reverse associations
- display reverse associations
- display summary of associated issues (forward and reverse)
associations of issues in progress
associated issues in progress
hide association property if no associated issue
compute and display reverse associations
fix lost summary in addEntry
remove a DEBUG that introduced a bug
fix reverse associations

fix also multiple select posting with empty selection (use a hidden
input)
add summary in associated issues
fix JSON export for trigger when non official properties are present
fix tr and td alignment in properties table and form