fix image uploads on s3/imgur

[ZogStriP]

Meta: Storing images in the cloud

Most of the work was done on S3 uploads. They will now work properly when using (or not) a region and will automagically create the bucket if it doesn't already exists.

Imgur mostly got a lifting and a few bug fixes (all props goes to @benbeltran).

• Changed a few site settings
  • removed unused imgur_client_secret
  • updated imgur_endpoint to use https
  • removed default for s3_region as it is no longer needed (and was wrong)
• Refactored upload.rb model and extracted all the methods into their own class
  • s3 for handling uploads to s3 buckets
  • local_store for handling uploads to a local directory
  • The idea is that these classes handles uploads and returns the url of the uploaded file.
• Created a new SiteSettingMissing exception that is used to hopefully provide better error log message whenever a site setting is missing (only used in s3.rb and imgur.rb for now)
• Updated the tests accordingly to the changes

[discoursebot]

You've signed the CLA, ZogStriP. Thank you! This pull request is ready for review.

[SamSaffron]

:) awesome