Add a share page

[wilzbach]

This adds a custom share page and tries to tackle #63. Currently the page looks ugly, so see this as a WIP.

Please note that the JS used is optional and just sets the size of the window opened. It works well without JS and no external (tracking) js is loaded. Also I am not sure anymore whether providing a separate page is really the way to go. What is your impression @CyberShadow?

[CyberShadow]

I meant the page solely as a fallback when JS is off, but maybe that is too much work for it.

No popup windows, please, everyone hates them. By "popup" I mean the HTML kind, e.g. like here on GitHub when you click the "add reaction" button. There is one already in DFeed to go to a specific page on the thread overview bar.

[CyberShadow]

famfamfam icons predate the new "share" icon that's universal now...