hijack nextbutton on preview page and rename it to Submit!

Summary:
it's very awkward when you don't know when a change happens or
not, so suitably label the next button so it actually says submit when
it will trigger the report submission.

CHANGELOG: Buttons are more suitably labled when bug reports are submitted

Test Plan: file bug report -> preview page has submit button -> going back it is not submit -> clicking submit it also turns back to next (albeit disabled)

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22289