First part of slogan API

Randomly return a tag, that’s it so far.
Implemented weighted random tags
Add information to metadata in API

Required for the sloganerator
Fixes #71

As in the sloganerator
The `startsWith` thing is getting confusing
Implement slogans on the tag view page
Implement historical remarks on the tag view page
Extended the API to give slogans
The tag suggestion for slogans is aware of slogans

At first we shouldn’t resuggest slogans too often.
Put sloganerator on the front page
Fixed a weird bug in recent comments

It’s not weird in hindsight. Just silly.
Add license stuff to comment input
Fixes #69, and is related to #65

Updated XyJax to latest version, that allows for CDN use of MathJax. Hence deleted MathJax.

Remark that it is now *required* to change the last line of `js/XyJax/extensions/TeX/xypic.js`! See the documentation for XyJax.
Make sure the localStorage works across pages
Merge branch 'refs/heads/slogans'