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

2 - Flash after clicking on export

3 - Email received with download link

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)

5 - After delete flash

Import

1 - New project page with new gitlab export option

2 - Next step importing - choosing a file

3 - Import in progress

4 - Import successful