Core Trac should allow marking tickets as duplicates without specifying a number

[SergeyBiryukov]

Currently, when you mark a ticket as duplicate without specifying a number (e.g. because the other ticket is on another Trac instance, see ​#WP12336 for example), two things happen:

1. The ticket will be closed without a resolution.
2. You have to set resolution separately.

We should be able to do this in a single step.

[nacin]

It seems like:

• Not entering a ticket number is a bug in ​https://github.com/nacin/TracDuplicates. An error should be issued.
• Bug gardeners should be allowed to choose "duplicate" from the dropdown where it once was.
• A plugin such as ​http://trac-hacks.org/wiki/TicketMoverPlugin could help us in the long run.

The nice thing is it is an easy query to find recently closed tickets that manage to not have a resolution, so this is not corrupting our data.

[SergeyBiryukov]

On the other hand, we don't often move tickets between Trac instances, so perhaps we can leave everything as is.

[SergeyBiryukov]

Related: #33