Compress cache to save space in localStorage

* Make use of lz-string plugin to compress cache's JSON, since it has a
very low entropy. Compressed  JSON is now at least 3 times smaller than
original. Naive renaming of properties to a 1-character analogs gives
only 35% of economy.
Example: initial 151Kb JSON was reduced to only 100K that way. Compare
this with LZ-compression, which reduced the file to 39Kb.
* Fix excessive quotation mark in default.htm.