Replace some icon to octicon

- replace some of the non-octicon to octicon.
- adjust the color of octicon on the button.

modified icon is as follows.

- .icon-home          -> .octicon-home
- .icon-time          -> .octicon-clock
- .icon-ok            -> .octicon-check
- .icon-lock          -> .octicon-lock
- .icon-envelope      -> .octicon-mail
- .icon-pencil        -> .octicon-pencil
- .icon-remove-circle -> .octicon-x
- .icon-check         -> .octicon-clippy
- .icon-calendar      -> .octicon-calendar
- .icon-cog           -> .octicon-gear
- .icon-th-list       -> .octicon-list-unordered
- .icon-trash         -> .octicon-trashcan
- .icon-arrow-right   -> .octicon-arrow-right
- .icon-retweet       -> .octicon-git-compare
- .icon-comment       -> .octicon-comment