Allow re-adding of previously deleted contact when preparing sequencing run

Users can add and remove contacts when setting up a sequencing run on the prepare sequencing run page. Once a contact is added, their (+) button in the contact list is disabled so that they can't be added again--all good. However, for ONLY SOME contacts, if that contact is then removed, the button isn't re-enabled, meaning they can't be added back later:

(Adding a contact back later is a reasonable action one wants the contacts to be in a particular order and realizes they have been entered in the wrong order.)

This appears to affect contacts that are NOT listed by their email: for example, Jon Sanders (listed in the contact list by name) cannot be re-added after deleting, whereas jshorens@gmail.com (listed in the contact list by email) CAN be re-added after deleting. I assume this is linked to the fact that the existing `removeContacts` method in `sequencing.html` re-enables contacts by their emails:

`$('[id="addBtnContact' + email + '"]').prop('disabled', false);`

... which presumably is breaking down in some way for the users not listed by their email.

This is irritating but not a show-stopper because it is probably a rare use-case, and because it can be worked around by abandoning the current effort of filling out the sequencing run form (which is minimal), leaving the page, coming back to it, and starting over.