chore: stop treating dirs as having size 128 (#10750)

Resp. directories for the database, and both dirs and symlinks for
`FileInfo.FileSize`. Instead handle it in the UI progress percentage. We
even already have special cases there for deletions, might as well
handle directories just like any other zero-sized needed item there.

This went through the very thorough testing of running it on my laptop,
the migration was applied and it seemed to be working fine after.

---------

Signed-off-by: Simon Frei <freisim93@gmail.com>

lib/protocol/protocol_test.go

@@ -464,7 +464,7 @@ func TestCheckConsistency(t *testing.T) {
 			fi: FileInfo{
 				Name: "foo",
 				Type: FileInfoTypeDirectory,
-				Size: SyntheticDirectorySize,
+				Size: deprecatedSyntheticDirectorySize,
 			},
 			ok: true,
 		},
@@ -492,7 +492,7 @@ func TestCheckConsistency(t *testing.T) {
 				Name:          "foo",
 				Type:          FileInfoTypeSymlink,
 				SymlinkTarget: []byte("bar"),
-				Size:          SyntheticDirectorySize,
+				Size:          deprecatedSyntheticDirectorySize,
 			},
 			ok: false,
 		},