Ability to delete all branches that have been merged since a given date
### Description
At the moment we have a Delete button to delete **all** the branches that have been merged to master.
However, this action is all or nothing. What if we could only delete branches older than a specific date to allow a fine grain management of the branches? Companies might want to keep at least one year of branches, merged or not, in their projects, and remove the rest.
### Description
* When clicking on the Delete merged branches button, display a panel asking
* Delete all the branches
* OR
* Delete all branches before DATE
issue