Add tags for projects

When i use
```ruby
image_tag(project.preview_image.image.url(:thumb)))
```
and project has no images, it raises error
`Undefined method image.for nil:Nil Class`
so i need to check if the project have an image.
And when i just build it using
```ruby
@project.preview_image.build
```
and use image tag on it, it renders no picture. So, I also need to check if the preview_image persisted.