[TASK] UI: Make base font consistent (RT-2747)

Replace the sans-serif default font with OpenSansRegular for all otherwise unstyled UI parts, drop unnecessary specific overrides.

- Before:
![2747-before](https://cloud.githubusercontent.com/assets/6348321/5183660/5cf97aa0-74b1-11e4-8373-8679403c46c6.jpg)

- After:
![2747-after](https://cloud.githubusercontent.com/assets/6348321/5183665/67c9ccfa-74b1-11e4-9ef4-26913e8bf3fe.jpg)

@mtrippled