Add a right and left border on issues/MRs index page

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Description

Currently, the issues and MRs index page feels a bit naked without borders.

We'd need to add something like border-right: 1px #e5e5e5 solid; and border-left: 1px #e5e5e5 solid;.

Here's a mockup of what it looks like with them:

screenshot-gitlab.com_2016-09-12_19-18-26

screenshot-localhost_3000_2016-09-12_19-11-01

Edited by 🤖 GitLab Bot 🤖