Fix non-alphanumeric characters in flag names

[willbarton]

As @chosak notes in #73, the admin URL patterns prevented creating flags that didn't match [\w\-]. This change fixes that by switching from re_path to path for those patterns, simplifying them and making them more readable.

Fixes #73

[chosak]

At long last my dream of creating a flag named 🎏 has come true...

One issue - with this change you can create a flag with a / in it, like foo/bar, but then when you get redirected to the index view you get

django.urls.exceptions.NoReverseMatch: Reverse for 'flag_index' with arguments '('foo/bar',)' not found. 1 pattern(s) tried: ['admin/flags/(?P[^/]+)/\Z']

Granted, this issue exists on main as well. What about changing the flag name form here to use a Django SlugField? Sadly this would prevent the use of emoji in flag names but would ensure that the slug-name-based URL patterns never break weirdly.

[chosak]

Thankfully one can still get creative: