Split IssueURL into IssueURL and IssueCommentURL.

Split PullRequestURL into PullRequestURL and PullRequestCommentURL.

Add PullRequestReviewURL and PullRequestReviewCommentURL.

Factor out Router from github.com/shurcooL/notifications/githubapi
package, so it can be more easily imported both by notifications/githubapi
and events/githubapi.

Add context parameter to enable some request-scoped flexibility over
routing.