Experimental UI for exporting and importing a project


Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/3050

Screenshots of both the export and import processes:

## Export
1 -  Project settings

 ![Screen_Shot_2016-06-16_at_15.29.27](/uploads/ec59113dae9132e594b79289e3598f5c/Screen_Shot_2016-06-16_at_15.29.27.png)

2 -  Flash after clicking on export

![Screen_Shot_2016-06-16_at_15.29.47](/uploads/02f20d1500de4e0c9693218f9fb2c414/Screen_Shot_2016-06-16_at_15.29.47.png)

3 - Email received with download link

![Screen_Shot_2016-06-16_at_15.36.19](/uploads/0f7e0a74125d9f1fa067eb52104397a5/Screen_Shot_2016-06-16_at_15.36.19.png)

4 - The project settings export screen changes so we can either delete the file or download it again (it won't generate a new export, unless we delete it first)

![Screen_Shot_2016-06-16_at_15.28.43](/uploads/073f87cc751a857eac94e55d1d0c4ef9/Screen_Shot_2016-06-16_at_15.28.43.png)

5 - After delete flash

![Screen_Shot_2016-06-16_at_15.29.10](/uploads/e80341aebcaed8f7713868793fda2b92/Screen_Shot_2016-06-16_at_15.29.10.png)

## Import
1 - New project page with new gitlab export option

![Screen_Shot_2016-06-16_at_15.31.25](/uploads/246e823a52c5b0216354c4f5321f846b/Screen_Shot_2016-06-16_at_15.31.25.png)

2 - Next step importing - choosing a file

![Screen_Shot_2016-06-16_at_15.32.23](/uploads/f91e72f68cc844577a0fc1935e3936d3/Screen_Shot_2016-06-16_at_15.32.23.png)

3 - Import in progress

![Screen_Shot_2016-06-16_at_15.32.48](/uploads/41c774c0c03a91b60cd220ce77cab8e6/Screen_Shot_2016-06-16_at_15.32.48.png)

4 - Import successful 

![Screen_Shot_2016-06-16_at_15.32.54](/uploads/337f9a07779999d00232f7ac61ed362b/Screen_Shot_2016-06-16_at_15.32.54.png)



See merge request !4012