Edit labels from issues and merge requests show page.

The idea of this feature is to have the possibility of editing labels from the issues and merge requests show pages.

#### **User Interface**

The implementation consists of a modified UI for issues and merge requests show page:

- the project's labels which aren't selected are displayed in a dropdown between Asignee and Milestone
- the selected labels are shown underneath the issue/merge request box. They can be removed from there using the remove image which is contained in the left area of the label

Screenshot before:
![before-issue-page](https://cloud.githubusercontent.com/assets/4246830/4625784/6592ad38-537a-11e4-94b3-a6d2d3fee466.png)
Screenshot after:
![after-issue-page2](https://cloud.githubusercontent.com/assets/4246830/4625796/7996e614-537a-11e4-903f-d51f71b5ee5f.png)

#### **Implementation**

- selecting a label from dropdown will add it to the issue/merge request through Javascript and the updated label's list will be shown underneath the issue/merge request box
- in order to remove a label, a backend modification was required. The partial created for these labels contains two links: the first removes the label and the second has the basic functionality – to filter by label.

#### **Testing**

A Spinach feature was written to test the functionality for the issue, given the fact that it is similar to the merge request functionality.  
Prefer single-quoted strings when you don't need string interpolation or special symbols.