Replace window.confirm() with styled modal to confirm credentials overwrite

[calebtuttle]

Low priority.

In store-credentials, use a modal instead of the default window.confirm() to ask user whether they would like to overwrite the credentials they already have from the given issuer.

Modal should include:

• A question / warning message (e.g., "Are you sure you want to overwrite credentials from ? This action cannot be undone.")
• The current credentials from the issuer (the ones that would be overwritten)
• The new credentials from the issuer