Refactored generate_palettes in PaletteSet model

* Replaced if/else control flow with guard condition
* Also memoized the Tumblr client by assigning it to an instance
  variable
Remove non-error flash from PaletteSetsController

* Remove flash[:alert] = "No photo posts foudn to generate palettes
  from" out of PaletteSetsController, and from the flash messages in
  general
  * Instead display the message in the body of the PaletteSet#show view,
    in lieu of palettes
Refactored PaletteSets#show view
Update .gitignore