Fix regression in new confirmation Dialog

Summary:
Fix a JavaScript regression encapsulating the problematic part into an if.

Other minor changes:

• dedicate a variable for the confirmation messages to improve i18n in the future (but also to avoid 80 characters and make lint happy)
• replace confirm with window.confirm (to make lint happy)

Ref T15034
Ref D25015

Test Plan:

• surf on your local Phorge
• no JavaScript errors in console

Reviewers: bekay, Ekubischta, O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15034

Differential Revision: https://we.phorge.it/D25076