Backwards compatibility and missing updates for project tags

[corneliusboehm]

Features

• Add backwards compatibility update when loading project config
  • Setup dictionary of project tags, update class tags and translate existing annotations
  • Check for all other project settings that have been added later
• Treat background as persistent default label
• Correctly handle tag indices that might have gaps (when tags have been removed), especially when building label2int dictionary
• Save max_tag_index so that a removed tag can not be overwritten by a new one
• Reset annotations for tags that have been removed
• Let logreg work on an arbitrary number of tags

Smaller Changes

• Don't show "This tag already exists" when leaving a tag name unchanged
• Flip tag name and index in project tags dictionary for easier processing
• Sort tags by value
• Remove duplicate FileNotFoundErrors
• Move buttons on same line with edit field
• Reset class name when editing is cancelled (similar to how it was done for tags)
• Directly send submitted forms for saving/removing project tags to the backend