This request makes "Your last update" of the extension info shown when
clicking more info triangle mark. The first installed date of the
extension may not be necessary for local site admin work.

':' is the field delimiter in the authplain flat text
conf/users.auth.php file, but it's also used as an internal delimiter
for the 'mediawiki' password hash format. Currently using this hash
format corrupts the file

This change escapes ':' as '\:' in any field in the users.auth.php
file, and any '\' as '\\'.

Also adds test cases for escaping modes.