MR banner & flash notices updates

### What does this MR do?
* Moves the create MR banner below subnav
* Changes MR banner background to white
* Moves flash notices underneath subnav
* Adds container within flash notices to align with content

Closes #22767 
### Why was this MR needed?
It looked a bit out of place between the two navs & had unneeded spacing.

### Screenshots (if relevant)
Before:   
<img src="/uploads/c1676a1923893a7ba7e1c6d6a50d4ea6/Screen_Shot_2016-09-09_at_7.17.31_PM.png" width="800px">
After:   
<img src="/uploads/74a2d6c8c43b9a15cd535a2a7adb2685/Screen_Shot_2016-09-28_at_3.41.29_PM.png" width="800px">

Before:   
<img src="/uploads/285e3de31326c48de43bb48abd4d907c/Screen_Shot_2016-09-29_at_10.57.55_AM.png" width="800px">
After:    
<img src="/uploads/0877e3e45755e5ca36694c52ad2e6198/Screen_Shot_2016-09-29_at_10.56.04_AM.png" width="800px">

With both alert and broadcast message:
![Screen_Shot_2016-09-29_at_11.06.51_AM](/uploads/ba3963a73b5c82940533c369ec54ec50/Screen_Shot_2016-09-29_at_11.06.51_AM.png)


See merge request !6581