Add full backup method

[sissbruecker]

Adds a new full_backup CLI command to create a full backup of linkding's data folder. That includes:

• The SQLite database
• The assets folder, which currently only contains HTML snapshots of bookmarked websites
• The favicons folder which contains downloaded favicons

The backup docs have been updated to promote this as the default backup method. The previous backup command that only creates a backup of the database has been deprecated.