Custom html elements in the right sidebar

[fekete-robert]

This PR adds hooks and placeholder files to include custom HTML elements in the right sidebar. For example, it can be used to show images or banners for events like here:

To use it, add your HTML code to the layouts/partials/hooks/custom-right-sidebar-elements-top.html or layouts/partials/hooks/custom-right-sidebar-elements-bottom.html file in your project directory.

(I haven't updated the userguide yet because I wasn't sure where to add this info.)

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