Validation of media files

[iqqmuT]

Support for files named checksums.md5 that are stored into USB sticks. With those files backend validates that files work correctly.

• Add Validate Files button to UI
• It opens a modal dialog where is a progress bar and Start button
• Start triggers backend to scan through all directories
• If backend finds a file named checksums.md5, it add those files into list
• After all files are listed, backend runs md5sum --check checksums.md5 for found files
• Backend updates progress bar
• If some files are invalid, errors are rendered