Rewrote the data loading logic

* Data is much more detailed now
* Saved to a SQLite database
 -> No redownloading needed everytime a graph is plotted
 -> Adds the possibility to create long-term statistics
fix update_db
use SQLite db for simple charts
fix NSFW checking
move url loading into a function
forgot to move one url loading segment
update tipped charts to use the db
remove download_data_* functions, they are obsolete now
catch KeyboardInterrupt
add some multithreading for getting tips
small bugfixes
add per subreddit stats
fail with incorrect amount of arguments; forgot to add new base.html