Update for 1.15

[ArchBlood]

Currently those that are using the beta version of 1.15 can't use the devtools due to humhub\widgets\MarkdownView class missing.

[ArchBlood]

@gevorgmansuryan, thanks for fixing this issue, but seems like everything else is also still broken, tested all options and it seems like the fields are broken and some of the example views are as well.