Grouping API request

We need the following API:

• Add an helper function to get the common prefix between two media
• Create a group from a list of media
• Add a Media::groupSimilar member function, that will group all media with similar names. It will only affect ungrouped media, regardless of the has_been_grouped flag
• Add a flag to reflect if the group has been renamed (since the UI will need to list them as group instead of as media)
• Ensure a group that was renamed doesn't get automatically renamed when adjusting the name length based on the common prefix
• Remove the unique constraint on group names
• Prevent user to specify an empty name when renaming
• Remove group getter by name, since there can now be 2 groups with the same name