Fix automatic scrolling of chat window

Around Chromium 60/61 google has deprecated use of
document.body.scrollTop in favour of
document.scrollingElement.scrollTop, which is why auto scrolling broke
with Qt 5.10 (shipped with Chromium 61).

https://bugs.chromium.org/p/chromium/issues/detail?id=766938

In addition to that there seems to be some name clash with the
scrollToBottom() function as the implementation inside Template.html
was never called. A simple rename fixed the problem.

Related Bug: https://bugs.kde.org/show_bug.cgi?id=388082

Reviewers: KDE Telepathy

Reviewed By: akulichalexandr

Subscribers: KDE Telepathy

Differential Revision: https://phabricator.kde.org/D16382