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

src/main/twirl/gitbucket/core/pulls/compare.scala.html

@@ -24,7 +24,7 @@
         <span class="label label-info monospace">@originRepository.owner:@originId</span> ... <span class="label label-info monospace">@forkedRepository.owner:@forkedId</span>
       </div>
       <div id="compare-edit" style="display: none;">
-        <a href="#" id="cancel-condition-editing" class="pull-right"><i class="icon-remove-circle"></i></a>
+        <a href="#" id="cancel-condition-editing" class="pull-right"><i class="octicon octicon-x"></i></a>
         @helper.html.dropdown(originRepository.owner + "/" + originRepository.name, "base fork") {
           @members.map { case (owner, name) =>
             <li><a href="#" class="origin-owner" data-owner="@owner" data-name="@name">@helper.html.checkicon(owner == originRepository.owner) @owner/@name</a></li>