focus email input line by default

Summary:
we already (re)set the focus on failed login attempt but neglected to do
it when showing the page.

BUG: 418309
FIXED-IN: 5.18.3

Test Plan:
- empty kwallet
- try to report bug
- dialog appears and has focus on email input
- go back
- go forward
- still focus on email input
- provide wrong credentials
- again focus on email input + error message

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27776