Web UI style fixes: about dialog cannot be opened in the startup page

[alterhu2020]

Fix these issues:

• When you startup the locust web page first time, in the main page, we cannot open the about page when click the bottom link "About"
• In the "Start new load test" and "Edit running load test" page, the top left close button is not formatted correct(disorderred) , so I corrected the close link position to the same as in the "About" dialog page. see the following screenshot:

1. Modified screenshot, which can show the "About" dialog in the startup page:
   

2. The "close" button in the "Start new load test" dialog page:
   

3. The "close" button in the "Edit running load test" dialog page:
   

[cyberw]

Good stuff, thanks!