Improve search page

Fixes #1270

* added pagination support
* filter results by type
* when switch between context - search params stays
* redesign comment search results
* search Issues and MR by description too

## Screenshots:

![Screenshot 2014-08-27 09.56.18](https://dev.gitlab.org/uploads/gitlab/gitlabhq/2053220cd4/Screenshot_2014-08-27_09.56.18.png)

![Screenshot 2014-08-27 09.56.00](https://dev.gitlab.org/uploads/gitlab/gitlabhq/726c764da4/Screenshot_2014-08-27_09.56.00.png)

See merge request !1043