Shorten long quotes

[Treora]

Quoting a longer text now creates huge URLs, which could easily be shortened while remaining uniquely resolvable. The most traditional and human-readable approach would be ellipsis: "ipsum dolor ... diam lectus.".

Possibly extra information could be added to increase robustness and verify the contents are correct. A hash of the string, or maybe just the length of the omitted part. I feel however that robustness to text modifications should be accomplished at a higher (document-)level, and making sure that the quote identifier was unambiguous in the first place is a matter that must be solved generally (issue #1; disambiguation could in this case be achieved by adding prefix/suffix as well as by removing less quoted words).

[Treora]

Implemented in 4618e55.