Task/t UI 468 add share button in file browser

[Mingyu-Wei]

Overview:

Add a Share button in file browser for non-authenticated users to download files/folders

Related Github Issues:

#468

Summary of Changes:

1. Added a ShareModal where users can specify allowed uses and valid seconds, and when users click 'Generate Link', the modal will display a shareable link. The user can easily click the Copy button to copy the link to clipboard

2. Display the Share button in the Toolbar

Testing Steps:

1. Select one file or directory from Files
2. Click the 'Share' button
3. Specify the allowed uses and valid seconds
4. Click the 'Generate Link' button
5. Click the 'Copy' button
6. The link should allow non-authenticated users to download the shared files (up to allowed uses times)