Added witness url + remaining votes

[aaroncox]

Spent a little time this evening tinkering with the witnesses page again. I've made some modifications including:

• The witness URL has been added as a link to the right most column.
• A small description has been added to the top of the page with how many remaining votes you have left along with the maximum.
• I thought some visual distinction and columns would do it justice, so I converted it into a table.
• Mobile optimizations for the "additional witnesses" section.

I'm curious to know what other type of basic information should be added to this page. I choose not to add the price feed, misses, blocks and more advanced user information just to help avoid confusion.