Admin can scale, crop, order images within a portfolio, and arrange them within a grid

- [x] Refactor the `FlickrImagesController` and extract a RESTful `PortfoliosController`
- [x] Portfolios display in a more dynamic grid (based on the Bootstrap grid) and aren't constrained to the 150px thumbnails from Flickr
- [x] Images are loaded in the most fitting size by default, and are dynamically reloaded with fade-over (on multiple layers, highest resolutions topmost, and stored in the `FlickrImageSizes` model) from Flickr when the browser is resized
- [ ] The PortfoliosController has an edit and an update functionality that allows for images to be placed in the grid with (mandatory) JS - an arbitrary number of rows and columns for each image, arbitrary starting point and scale for the image with `overflow: hidden`, aspect ration 3:2 per grid cell
- [ ] The image's positions are stored within the expanded `flickr_images_flickr_tags` (?) table instead of the `flickr_images` table, and are auto-updated when the grid positions are changed
- [x] Portfolios have images "jump in line" for lower-res screens, responsive style