Add new resolution for banner image and compression for jpg

[armandfardeau]

🎩 What? Why?

Banner image were low quality because of improper resolution configuration: 1000px x 200px. Causing an unpleasant user experience on high resolution devices (retina screen).

New resolution setting is : 1200px x 600px.

This pull request add a new feature too : ✨ images compression ! ✨
Set on 60 % on banner image to increase performance.
Jpg can be compressed from 50% to 70% without any visual significant difference.

🤔 How to use it:
add process quality: 60 to your uploader
Example:

module Decidim
  # This class deals with uploading banner images to ParticipatoryProcesses.
  class BannerImageUploader < ImageUploader
    process resize_to_limit: [1200, 600]
    process quality: 60
  end
end

💯 How to update already existing images?
Open your rails console
rails c
paste the following :
Decidim::ParticipatoryProcess.find_each { |process| process.banner_image.recreate_versions! if process.banner_image? }

⚠️ Always backup before ! ⚠️

📌 Related Issues

• Fixes Banner image not clear #148

📷 Screenshots (optional)

Before :

After :

After, with compression enabled :

👻 GIF

[juliesimon]

Seems like an awesome update for me @armandfardeau !!
I say yes :)

From a technical point of view, I'd rather have @moustachu's opinion before merging the PR

[armandfardeau]

What's really awesome is the ability to compress jpeg. That's mean better resolution at the same weight.

As a reward i suggest a stickers "#portdelaturballe" 😆