"Add" button remains greyed out when pasting a tag name in the "Manage official bug tags" page

[Pierre Equoy]

Steps to reproduce
==================

1. Go to a project's official bug tags management page:

https://bugs.launchpad.net/<project>/+manage-official-tags

2. In the "Add a new official tag:" text field, paste a tag name (e.g. "example-tag") using Ctrl+V

Expected result
==================

The tag is pasted and user can click on "Add" to add it

Actual result
==================

The tag is pasted but the "Add" button remains greyed out

Workaround
==================

As soon as another character is input in the text field (e.g. space button), the "Add" button becomes active.

I suspect the root cause is that there is some JS to toggle the "Add" button state to make sure at least two characters have been entered. When content is pasted, this condition is not triggered and the "Add" button remains inactive.