public url regex default

[will-moore]

What this PR does

Updates the public url_filter default value so that it matches nothing.
This means users will need to set this, instead of relying on the default value.

Testing this PR

1. Set public user as on https://docs.openmicroscopy.org/omero/5.3.3/sysadmins/public.html#public-user without setting the url_filter.
2. Should find that NO urls are publicly available (will get redirected to login page)
3. Setting the url_filter to e.g. "^/(webgateway)" should still work OK.

Related reading

See discussion at #5315 about /api/ urls being available with the default url_filter setting.
The "regex that matches nothing" in this PR came from: https://stackoverflow.com/questions/940822/regular-expression-syntax-for-match-nothing for

[kennethgillen]

Is it worth stating that this is a regular expression? e.g. "Set a regular expression for URLs". filter could be some custom syntax, whereas calling it a regular expression tells the user the syntax which is expected in the string. Or do all Python devs know that the wee r at the start of the string signifies a regex (if indeed it does?)?

[kennethgillen]

Perhaps a comment, such as: r'(?#This regular expression matches nothing)a^',
syntax cf http://www.regular-expressions.info/refadv.html

[will-moore]

Thanks. Updated both.

[rgozim]

With a public user set, I tested a number of URLs from my history and they were all redirected back to the login page, as intended.

I also tested the 'a^' regex on a couple of online regex tools and there were no issues.

Seems good to me.