Cleanup layout of personal access tokens settings page

(This was identified in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20332#note_88387746)

Description

There are some styling issues with the personal access tokens page. Let's make it better! 🚀

Proposal

Here are the two significant issues that were identified:

• Convert layout to single-column in order to give more space to the personal access tokens table. (See this discussion)
• Change the "no personal access token" message styling. Right now it looks strange. It should probably be just a pragraph. (See this comment)
• Add .table.responsive-table to personal access token table. (See this comment)