escape backslash in find&replace

https://anki.tenderapp.com/discussions/ankidesktop/35903-bug-find-and-replace-breaks-when-replacing-with-latexmathjax-commands

anki/find.py

@@ -492,6 +492,7 @@ def findReplace(col, nids, src, dst, regex=False, field=None, fold=True):
     # find and gather replacements
     if not regex:
         src = re.escape(src)
+        dst = dst.replace("\\", "\\\\")
     if fold:
         src = "(?i)"+src
     regex = re.compile(src)