Migrates the following columns from MB to Bytes:
ApplicationSettings#repository_size_limit
Namespace#repository_size_limit
Project#repository_size_limit
And also convert from MB to Bytes on the proper user inputs.
Doing this we'll be able to fix #1541 (closed), and store Bytes instead MB's, which is a good improvement.
Backports: gitlab-ce!8616, gitlab-ce!8665
Closes #1541 (closed)