New design for merged branches deletion confirmation

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/28849

Path	Current Screenshot	New Title	New Description	New Buttons
project/repository/branches(merged)		Delete all merged branches?	You’re about to permanently delete [Branch count] branches that were merged into ‘[Default branch name]’. Once you confirm and press Delete merged branches, it cannot be undone or recovered. To confirm, type [Branch count] branches	[Cancel] [Delete merged branches]

Existing discussion

(https://gitlab.com/gitlab-org/gitlab-ce/issues/28849#note_52046368)

Copy for scenarios while a user is not able to delete the branch:

A branch doesn't exist (Maybe it got deleted by some user during the execution of the button press )

*[Branch name]* doesn't exist. Another project member may have deleted it already.

A user doesn't have push access to a repo an issued the command somehow

You do not have permission to delete this branch.

A default error which results in a error message that says Failed to remove branch

Failed to remove branch. Please try again.