sharing: UI improvements

In the File/Media Sharing dialogs:

• Add a explanatory text to the "File Sharing" and "Media Sharing" rows explaining how these sharing features work
• Remove the AdwPreferencesPage descriptions, since it's redundant with the explanatory text provided in the "File Sharing" and "Media Sharing" rows
• And add a row to display and copy the DAV address and add ".local" at the end of the hostname (in the File Sharing dialog)
• Add notice about shared files not being encrypted (closes #2972)

Reference: Teams/Design/settings-mockups@cfb7cae4

File Sharing	Media Sharing
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