Add a cancel button in liaison form

`owner:esanchez@yaco.es` `resolution_fixed` `type_enhancement`   |    by esanchez@yaco.es

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Add a cancel button in the liaison form so the user can discard the form and return to the liaison list. 

Warn the user that the data entered will be lost.

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_Issue migrated from trac:379 at 2022-03-04 01:02:24 +0000_