Don't let users view past the end of contigs

[akmorrow13]

Previously, the browser could scroll past the end of contigs. We now grab the number of bases from the current contig being viewed, and restrict viewing to this region. This required a substantial rebase of viz tests, that previously used a twobit file with short contigs. We now use a fake twobit file so all regions can be accessed during testing.

TODOs:

• fix slider in controls.js

[coveralls]

Coverage increased (+0.5%) to 91.32% when pulling cabe15c on akmorrow13:minor_fixes into 7cc3016 on hammerlab:master.